From 0494bf3105f7b4ffee00c9268673531d673aec64 Mon Sep 17 00:00:00 2001
From: Sam Atkins <sam@ladybird.org>
Date: Wed, 6 Nov 2024 16:43:57 +0000
Subject: [PATCH] Tests: Import all WPT css/css-nesting tests

A few are skipped for now:
- A few ref tests fail
- Crash tests are not supported by our runner and time out
- top-level-is-scope.html crashes and needs further investigation
---
 .../conditional-properties-ref.html           |  33 +++
 .../css-nesting/conditional-rules-ref.html    |  64 +++++
 .../css/css-nesting/has-nesting-ref.html      |   8 +
 .../css/css-nesting/implicit-nesting-ref.html |  27 ++
 .../nest-containing-forgiving-ref.html        |  19 ++
 .../supports-is-consistent-ref.html           |   5 +
 .../css/css-nesting/supports-rule-ref.html    |  23 ++
 .../ref-filled-green-100px-square-only.html   |   4 +
 .../css-nesting/conditional-properties.html   |  34 +++
 .../css/css-nesting/conditional-rules.html    |  75 ++++++
 .../css/css-nesting/has-nesting.html          |  21 ++
 .../css/css-nesting/host-nesting-001.html     |  23 ++
 .../css/css-nesting/host-nesting-002.html     |  22 ++
 .../css/css-nesting/host-nesting-003.html     |  26 ++
 .../css/css-nesting/host-nesting-004.html     |  25 ++
 .../css/css-nesting/host-nesting-005.html     |  24 ++
 .../css/css-nesting/implicit-nesting.html     |  82 ++++++
 .../nest-containing-forgiving.html            |  34 +++
 .../css-nesting/nesting-type-selector.html    |  18 ++
 .../css-nesting/supports-is-consistent.html   |  17 ++
 .../css/css-nesting/supports-rule.html        |  55 ++++
 Tests/LibWeb/TestConfig.ini                   |  15 ++
 .../css/css-nesting/block-skipping.txt        |  11 +
 .../implicit-nesting-ident-recovery.txt       |  12 +
 .../css-nesting/implicit-nesting-ident.txt    |  11 +
 .../css/css-nesting/invalid-inner-rules.txt   |  12 +
 .../css/css-nesting/invalidation-001.txt      |  11 +
 .../css/css-nesting/invalidation-002.txt      |  11 +
 .../css/css-nesting/invalidation-003.txt      |  11 +
 .../css/css-nesting/invalidation-004.txt      |  11 +
 .../nested-declarations-cssom-whitespace.txt  |  12 +
 .../css-nesting/nested-declarations-cssom.txt |  23 ++
 .../nested-declarations-matching.txt          |  21 ++
 .../nested-rule-cssom-invalidation.txt        |  11 +
 .../css/css-nesting/nesting-layer.txt         |  11 +
 .../wpt-import/css/css-nesting/parsing.txt    |  43 ++++
 .../serialize-group-rules-with-decls.txt      |  26 ++
 .../top-level-parent-pseudo-specificity.txt   |  11 +
 .../LibWeb/Text/input/wpt-import/common/gc.js |  52 ++++
 .../css/css-nesting/block-skipping.css        |   6 +
 .../css/css-nesting/block-skipping.html       |  17 ++
 .../css-nesting/delete-other-rule-crash.html  |  15 ++
 .../implicit-nesting-ident-recovery.html      |  29 +++
 .../css-nesting/implicit-nesting-ident.html   |  22 ++
 .../implicit-parent-insertion-crash.html      |  16 ++
 .../css/css-nesting/invalid-inner-rules.html  |  56 +++++
 .../css/css-nesting/invalidation-001.html     |  33 +++
 .../css/css-nesting/invalidation-002.html     |  33 +++
 .../css/css-nesting/invalidation-003.html     |  34 +++
 .../css/css-nesting/invalidation-004.html     |  30 +++
 .../nested-declarations-cssom-whitespace.html |  52 ++++
 .../nested-declarations-cssom.html            | 237 ++++++++++++++++++
 .../nested-declarations-matching.html         | 221 ++++++++++++++++
 .../nested-rule-cssom-invalidation.html       |  41 +++
 .../css/css-nesting/nesting-layer.html        |  52 ++++
 .../wpt-import/css/css-nesting/parsing.html   |  87 +++++++
 .../css/css-nesting/pseudo-part-crash.html    |  12 +
 .../css/css-nesting/pseudo-where-crash.html   |   9 +
 .../serialize-group-rules-with-decls.html     |  91 +++++++
 .../css/css-nesting/top-level-is-scope.html   |  30 +++
 .../top-level-parent-pseudo-specificity.html  |  19 ++
 .../css/support/delete-other-rule-crash.css   |   2 +
 62 files changed, 2098 insertions(+)
 create mode 100644 Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/conditional-properties-ref.html
 create mode 100644 Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/conditional-rules-ref.html
 create mode 100644 Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/has-nesting-ref.html
 create mode 100644 Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/implicit-nesting-ref.html
 create mode 100644 Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/nest-containing-forgiving-ref.html
 create mode 100644 Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/supports-is-consistent-ref.html
 create mode 100644 Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/supports-rule-ref.html
 create mode 100644 Tests/LibWeb/Ref/expected/wpt-import/css/reference/ref-filled-green-100px-square-only.html
 create mode 100644 Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/conditional-properties.html
 create mode 100644 Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/conditional-rules.html
 create mode 100644 Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/has-nesting.html
 create mode 100644 Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/host-nesting-001.html
 create mode 100644 Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/host-nesting-002.html
 create mode 100644 Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/host-nesting-003.html
 create mode 100644 Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/host-nesting-004.html
 create mode 100644 Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/host-nesting-005.html
 create mode 100644 Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/implicit-nesting.html
 create mode 100644 Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/nest-containing-forgiving.html
 create mode 100644 Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/nesting-type-selector.html
 create mode 100644 Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/supports-is-consistent.html
 create mode 100644 Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/supports-rule.html
 create mode 100644 Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/block-skipping.txt
 create mode 100644 Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/implicit-nesting-ident-recovery.txt
 create mode 100644 Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/implicit-nesting-ident.txt
 create mode 100644 Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/invalid-inner-rules.txt
 create mode 100644 Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/invalidation-001.txt
 create mode 100644 Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/invalidation-002.txt
 create mode 100644 Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/invalidation-003.txt
 create mode 100644 Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/invalidation-004.txt
 create mode 100644 Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/nested-declarations-cssom-whitespace.txt
 create mode 100644 Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/nested-declarations-cssom.txt
 create mode 100644 Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/nested-declarations-matching.txt
 create mode 100644 Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/nested-rule-cssom-invalidation.txt
 create mode 100644 Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/nesting-layer.txt
 create mode 100644 Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/parsing.txt
 create mode 100644 Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/serialize-group-rules-with-decls.txt
 create mode 100644 Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/top-level-parent-pseudo-specificity.txt
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/common/gc.js
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/block-skipping.css
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/block-skipping.html
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/delete-other-rule-crash.html
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/implicit-nesting-ident-recovery.html
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/implicit-nesting-ident.html
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/implicit-parent-insertion-crash.html
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/invalid-inner-rules.html
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/invalidation-001.html
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/invalidation-002.html
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/invalidation-003.html
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/invalidation-004.html
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/nested-declarations-cssom-whitespace.html
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/nested-declarations-cssom.html
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/nested-declarations-matching.html
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/nested-rule-cssom-invalidation.html
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/nesting-layer.html
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/parsing.html
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/pseudo-part-crash.html
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/pseudo-where-crash.html
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/serialize-group-rules-with-decls.html
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/top-level-is-scope.html
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/css-nesting/top-level-parent-pseudo-specificity.html
 create mode 100644 Tests/LibWeb/Text/input/wpt-import/css/support/delete-other-rule-crash.css

diff --git a/Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/conditional-properties-ref.html b/Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/conditional-properties-ref.html
new file mode 100644
index 000000000000..0285acbf33f9
--- /dev/null
+++ b/Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/conditional-properties-ref.html
@@ -0,0 +1,33 @@
+<!DOCTYPE html>
+<title>Properties in nested conditional rules</title>
+<link rel="author" title="Adam Argyle" href="mailto:argyle@google.com">
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/">
+<style>
+  .test {
+    background-color: red;
+    width: 100px;
+    height: 100px;
+    display: grid;
+  }
+
+  @media (min-width: 50px) {
+    .test-5 > div {
+      background-color: green;
+    }
+  }
+
+  @supports (display: grid) {
+    .test-10 {
+      background-color: green;
+    }
+  }
+
+  body * + * {
+    margin-top: 8px;
+  }
+</style>
+<body>
+  <p>Tests pass if <strong>block is green</strong></p>
+  <div class="test test-5"><div></div></div>
+  <div class="test test-10"><div></div></div>
+</body>
diff --git a/Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/conditional-rules-ref.html b/Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/conditional-rules-ref.html
new file mode 100644
index 000000000000..c4fabd672a46
--- /dev/null
+++ b/Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/conditional-rules-ref.html
@@ -0,0 +1,64 @@
+<!DOCTYPE html>
+<title>Conditional rules with nesting</title>
+<link rel="author" title="Adam Argyle" href="mailto:argyle@google.com">
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/">
+<style>
+  .test {
+    background-color: red;
+    width: 30px;
+    height: 30px;
+    display: grid;
+  }
+
+  @media (min-width: 10px) {
+    .test-5 > div {
+      background-color: green;
+    }
+  }
+
+  @media (min-width: 10px) {
+    .test-6 > div {
+      background-color: green;
+    }
+  }
+
+  @supports (display: grid) {
+    .test-10 {
+      background-color: green;
+    }
+  }
+
+  @layer {
+    .test-11 {
+      background-color: green !important;
+    }
+  }
+
+  @scope (.test-12) {
+    :scope {
+      background-color: green;
+    }
+  }
+
+  div {
+    container-type: inline-size;
+  }
+  @container (width >= 0px) {
+    .test-13 {
+      background-color: green;
+    }
+  }
+
+  body * + * {
+    margin-top: 8px;
+  }
+</style>
+<body>
+  <p>Tests pass if <strong>block is green</strong></p>
+  <div class="test test-5"><div></div></div>
+  <div class="test test-6"><div></div></div>
+  <div class="test test-10"><div></div></div>
+  <div class="test test-11"><div></div></div>
+  <div class="test"><div class="test-12"></div></div>
+  <div class="test"><div class="test-13"></div></div>
+</body>
diff --git a/Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/has-nesting-ref.html b/Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/has-nesting-ref.html
new file mode 100644
index 000000000000..5ab79b68f100
--- /dev/null
+++ b/Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/has-nesting-ref.html
@@ -0,0 +1,8 @@
+<!doctype html>
+<title>Nested has shouldn't match</title>
+<style>
+ul { background: green }
+</style>
+<ul>
+  <li>Bar</li>
+</ul>
diff --git a/Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/implicit-nesting-ref.html b/Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/implicit-nesting-ref.html
new file mode 100644
index 000000000000..0057a67fd037
--- /dev/null
+++ b/Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/implicit-nesting-ref.html
@@ -0,0 +1,27 @@
+<!DOCTYPE html>
+<title>Implicit nesting</title>
+<link rel="author" title="Steinar H. Gunderson" href="mailto:sesse@chromium.org">
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/">
+<style>
+  .test {
+    background-color: green;
+    width: 30px;
+    height: 30px;
+    display: grid;
+  }
+
+  body * + * {
+    margin-top: 8px;
+  }
+</style>
+<body>
+  <p>Tests pass if <strong>block is green</strong></p>
+  <div class="test"></div>
+  <div class="test"></div>
+  <div class="test"></div>
+  <div class="test"></div>
+  <div class="test"></div>
+  <div class="test"></div>
+  <div class="test"></div>
+  <div class="test"></div>
+</body>
diff --git a/Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/nest-containing-forgiving-ref.html b/Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/nest-containing-forgiving-ref.html
new file mode 100644
index 000000000000..8fb7c8674a53
--- /dev/null
+++ b/Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/nest-containing-forgiving-ref.html
@@ -0,0 +1,19 @@
+<!DOCTYPE html>
+<title>Nest-containing in forgiving parsing</title>
+<style>
+  .test {
+    background-color: green;
+    width: 100px;
+    height: 100px;
+    display: grid;
+  }
+
+  body * + * {
+    margin-top: 8px;
+  }
+</style>
+<body>
+  <p>Tests pass if <strong>block is green</strong></p>
+  <div class="test"></div>
+  <div class="test"></div>
+</body>
diff --git a/Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/supports-is-consistent-ref.html b/Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/supports-is-consistent-ref.html
new file mode 100644
index 000000000000..0eb8cd16a168
--- /dev/null
+++ b/Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/supports-is-consistent-ref.html
@@ -0,0 +1,5 @@
+<!DOCTYPE html>
+<title>@supports needs to be consistent with actual nesting support</title>
+<body>
+  <p>Test passes if this text is not red</p>
+</body>
diff --git a/Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/supports-rule-ref.html b/Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/supports-rule-ref.html
new file mode 100644
index 000000000000..1be71b225233
--- /dev/null
+++ b/Tests/LibWeb/Ref/expected/wpt-import/css/css-nesting/supports-rule-ref.html
@@ -0,0 +1,23 @@
+<!DOCTYPE html>
+<title>@supports with nesting</title>
+<link rel="author" title="Matthieu Dubet" href="mailto:m_dubet@apple.com">
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/">
+<style>
+  .test {
+    background-color: green;
+    width: 100px;
+    height: 100px;
+    display: grid;
+  }
+
+  body * + * {
+    margin-top: 8px;
+  }
+</style>
+<body>
+  <p>Tests pass if <strong>block is green</strong></p>
+  <div class="test"></div>
+  <div class="test"></div>
+  <div class="test"></div>
+  <div class="test"></div>
+</body>
diff --git a/Tests/LibWeb/Ref/expected/wpt-import/css/reference/ref-filled-green-100px-square-only.html b/Tests/LibWeb/Ref/expected/wpt-import/css/reference/ref-filled-green-100px-square-only.html
new file mode 100644
index 000000000000..82fcaa3b2aa4
--- /dev/null
+++ b/Tests/LibWeb/Ref/expected/wpt-import/css/reference/ref-filled-green-100px-square-only.html
@@ -0,0 +1,4 @@
+<!DOCTYPE html>
+<link rel="author" title="Morten Stenshorne" href="mstensho@chromium.org">
+<p>Test passes if there is a filled green square.</p>
+<div style="width:100px; height:100px; background:green;"></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/conditional-properties.html b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/conditional-properties.html
new file mode 100644
index 000000000000..36f3ac802ec1
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/conditional-properties.html
@@ -0,0 +1,34 @@
+<!DOCTYPE html>
+<title>Properties in nested conditional rules</title>
+<link rel="author" title="Adam Argyle" href="mailto:argyle@google.com">
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/">
+<link rel="match" href="../../../../expected/wpt-import/css/css-nesting/conditional-properties-ref.html">
+<style>
+  .test {
+    background-color: red;
+    width: 100px;
+    height: 100px;
+    display: grid;
+  }
+
+  .test-5 {
+    @media (min-width: 50px) {
+      background-color: green;
+    }
+  }
+
+  .test-10 {
+    @supports (display: grid) {
+      background-color: green;
+    }
+  }
+
+  body * + * {
+    margin-top: 8px;
+  }
+</style>
+<body>
+  <p>Tests pass if <strong>block is green</strong></p>
+  <div class="test test-5"><div></div></div>
+  <div class="test test-10"><div></div></div>
+</body>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/conditional-rules.html b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/conditional-rules.html
new file mode 100644
index 000000000000..507c0ac94a12
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/conditional-rules.html
@@ -0,0 +1,75 @@
+<!DOCTYPE html>
+<title>Conditional rules with nesting</title>
+<link rel="author" title="Adam Argyle" href="mailto:argyle@google.com">
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/">
+<link rel="match" href="../../../../expected/wpt-import/css/css-nesting/conditional-rules-ref.html">
+<style>
+  .test {
+    background-color: red;
+    width: 30px;
+    height: 30px;
+    display: grid;
+  }
+
+  .test-5 {
+    @media (min-width: 10px) {
+      & {
+        background-color: green;
+      }
+    }
+  }
+
+  .test-6 {
+    @media (min-width: 10px) {
+      background-color: green;
+    }
+  }
+
+  .test-10 {
+    @supports (display: grid) {
+      & {
+        background-color: green;
+      }
+    }
+  }
+
+  .test-11 {
+    @layer {
+      & {
+        background-color: green !important;
+      }
+    }
+  }
+
+  .test-12 {
+    @scope (&) {
+      :scope {
+        background-color: green;
+      }
+    }
+  }
+
+  div {
+    container-type: inline-size;
+  }
+  .test-13 {
+    @container (width >= 0px) {
+      & {
+        background-color: green;
+      }
+    }
+  }
+
+  body * + * {
+    margin-top: 8px;
+  }
+</style>
+<body>
+  <p>Tests pass if <strong>block is green</strong></p>
+  <div class="test test-5"></div>
+  <div class="test test-6"></div>
+  <div class="test test-10"></div>
+  <div class="test test-11"></div>
+  <div class="test test-12"><div class="test-12"></div></div>
+  <div class="test"><div class="test-13"></div></div>
+</body>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/has-nesting.html b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/has-nesting.html
new file mode 100644
index 000000000000..6efbac13f749
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/has-nesting.html
@@ -0,0 +1,21 @@
+<!doctype html>
+<title>Nested has shouldn't match</title>
+<link rel="help" href="https://drafts.csswg.org/selectors/#relational">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1864647">
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/9600">
+<link rel="match" href="../../../../expected/wpt-import/css/css-nesting/has-nesting-ref.html">
+<style>
+ul { background: green }
+
+li:has(strong) {
+  display: none;
+
+  :has(> &) {
+    background: red;
+  }
+}
+</style>
+<ul>
+  <li><strong>Foo</strong></li>
+  <li>Bar</li>
+</ul>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/host-nesting-001.html b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/host-nesting-001.html
new file mode 100644
index 000000000000..ea8b0b61707d
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/host-nesting-001.html
@@ -0,0 +1,23 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>:host and nesting (basic) </title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://drafts.csswg.org/css-nesting/#nest-selector">
+<link rel="match" href="../../../../expected/wpt-import//css/reference/ref-filled-green-100px-square-only.html">
+<p>Test passes if there is a filled green square.</p>
+<div id="host"></div>
+<script>
+  host.attachShadow({mode: "open"}).innerHTML = `
+    <style>
+      :host {
+        .nested {
+          width: 100px;
+          height: 100px;
+          background-color: green;
+        }
+      }
+    </style>
+    <div class="nested"></div>
+  `;
+</script>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/host-nesting-002.html b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/host-nesting-002.html
new file mode 100644
index 000000000000..11bc39ff0c24
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/host-nesting-002.html
@@ -0,0 +1,22 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>:host and nesting (bare declarations)</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://drafts.csswg.org/css-nesting/#nest-selector">
+<link rel="match" href="../../../../expected/wpt-import//css/reference/ref-filled-green-100px-square-only.html">
+<p>Test passes if there is a filled green square.</p>
+<div id="host"></div>
+<script>
+  host.attachShadow({mode: "open"}).innerHTML = `
+    <style>
+      :host {
+        @media (width >= 0) {
+          width: 100px;
+          height: 100px;
+          background-color: green;
+        }
+      }
+    </style>
+  `;
+</script>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/host-nesting-003.html b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/host-nesting-003.html
new file mode 100644
index 000000000000..6dd1af5fe9c7
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/host-nesting-003.html
@@ -0,0 +1,26 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>:host and nesting (combined with something else)</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://drafts.csswg.org/css-nesting/#nest-selector">
+<link rel="match" href="../../../../expected/wpt-import//css/reference/ref-filled-green-100px-square-only.html">
+<p>Test passes if there is a filled green square.</p>
+<div id="host"></div>
+<script>
+  host.attachShadow({mode: "open"}).innerHTML = `
+    <style>
+      .nested {
+        width: 100px;
+        height: 100px;
+        background-color: green;
+      }
+      :host(#not-host), #host {
+        .nested {
+          background-color: red;
+        }
+      }
+    </style>
+    <div class="nested"></div>
+  `;
+</script>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/host-nesting-004.html b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/host-nesting-004.html
new file mode 100644
index 000000000000..5d016e933e38
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/host-nesting-004.html
@@ -0,0 +1,25 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>:host and nesting (combined with something else, bare declarations)</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://drafts.csswg.org/css-nesting/#nest-selector">
+<link rel="match" href="../../../../expected/wpt-import//css/reference/ref-filled-green-100px-square-only.html">
+<p>Test passes if there is a filled green square.</p>
+<div id="host"></div>
+<script>
+  host.attachShadow({mode: "open"}).innerHTML = `
+    <style>
+      :host {
+        width: 100px;
+        height: 100px;
+        background-color: green;
+      }
+      :host(#not-host), #host {
+        @media (width >= 0) {
+          background-color: red;
+        }
+      }
+    </style>
+  `;
+</script>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/host-nesting-005.html b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/host-nesting-005.html
new file mode 100644
index 000000000000..f225a3811913
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/host-nesting-005.html
@@ -0,0 +1,24 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>:host and nesting (with pseudos)</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://drafts.csswg.org/css-nesting/#nest-selector">
+<link rel="match" href="../../../../expected/wpt-import//css/reference/ref-filled-green-100px-square-only.html">
+<p>Test passes if there is a filled green square.</p>
+<div id="host"></div>
+<script>
+  host.attachShadow({mode: "open"}).innerHTML = `
+    <style>
+      :host {
+        &::before {
+          display: block;
+          content: "";
+          width: 100px;
+          height: 100px;
+          background-color: green;
+        }
+      }
+    </style>
+  `;
+</script>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/implicit-nesting.html b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/implicit-nesting.html
new file mode 100644
index 000000000000..2567dba21f05
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/implicit-nesting.html
@@ -0,0 +1,82 @@
+<!DOCTYPE html>
+<title>Implicit nesting</title>
+<link rel="author" title="Steinar H. Gunderson" href="mailto:sesse@chromium.org">
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/">
+<link rel="match" href="../../../../expected/wpt-import/css/css-nesting/implicit-nesting-ref.html">
+<style>
+  .test {
+    background-color: red;
+    width: 30px;
+    height: 30px;
+    display: grid;
+  }
+
+  .test-1 {
+    > div {
+      background-color: green;
+    }
+  }
+
+  .test-2 {
+    .test-2-child {
+      background-color: green;
+    }
+  }
+  .test-2-child {
+    background-color: red;
+  }
+
+  .test-3-child {
+    background-color: red;
+  }
+  .test-3-child {
+    .test-3 & {
+      background-color: green;
+    }
+  }
+
+  .test-4 {
+    :is(&) {
+      background-color: green;
+    }
+  }
+
+  .test-5 {
+    :is(.test-5, &.does-not-exist) {
+      background-color: green;
+    }
+  }
+
+  .test-6 {
+    > .foo,.test-6-child,+ .bar {
+      background-color: green;
+    }
+  }
+
+  .test-7 {
+    > .foo, .bar, + .test-7-sibling {
+      background-color: green;
+    }
+  }
+
+  .test-8 {
+    > .foo, .test-8-child, + .bar {
+      background-color: green;
+    }
+  }
+
+  body * + * {
+    margin-top: 8px;
+  }
+</style>
+<body>
+  <p>Tests pass if <strong>block is green</strong></p>
+  <div class="test test-1"><div></div></div>
+  <div class="test test-2"><div class="test-2-child"></div></div>
+  <div class="test test-3"><div class="test-3-child"></div></div>
+  <div class="test test-4"></div>
+  <div class="test test-5"><div class="test-5"></div></div>
+  <div class="test test-6"><div class="test-6-child"></div></div>
+  <div class="test test-7" style="display:none"></div><div class="test test-7-sibling"></div>
+  <div class="test test-8"><div class="test-8-child"></div></div>
+</body>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/nest-containing-forgiving.html b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/nest-containing-forgiving.html
new file mode 100644
index 000000000000..f3593ae955cc
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/nest-containing-forgiving.html
@@ -0,0 +1,34 @@
+<!DOCTYPE html>
+<title>Nest-containing in forgiving parsing</title>
+<link rel="author" title="Steinar H. Gunderson" href="mailto:sesse@chromium.org">
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/">
+<link rel="match" href="../../../../expected/wpt-import/css/css-nesting/nest-containing-forgiving-ref.html">
+<style>
+  .test {
+    background-color: red;
+    width: 100px;
+    height: 100px;
+    display: grid;
+  }
+
+  .does-not-exist {
+    :is(.test-1, !&) {
+      background-color: green;
+    }
+  }
+
+  .does-not-exist {
+    :is(.test-2, :unknown(div,&)) {
+      background-color: green;
+    }
+  }
+
+  body * + * {
+    margin-top: 8px;
+  }
+</style>
+<body>
+  <p>Tests pass if <strong>block is green</strong></p>
+  <div class="test test-1"></div>
+  <div class="test test-2"></div>
+</body>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/nesting-type-selector.html b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/nesting-type-selector.html
new file mode 100644
index 000000000000..f5c27f3d998b
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/nesting-type-selector.html
@@ -0,0 +1,18 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>Nesting works with bare type selectors</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/">
+<link rel="match" href="../../../../expected/wpt-import//css/reference/ref-filled-green-100px-square-only.html">
+<style>
+:root {
+  div {
+    width: 100px;
+    height: 100px;
+    background: green;
+  }
+}
+</style>
+<p>Test passes if there is a filled green square.</p>
+<div></div>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/supports-is-consistent.html b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/supports-is-consistent.html
new file mode 100644
index 000000000000..e99932ee591a
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/supports-is-consistent.html
@@ -0,0 +1,17 @@
+<!DOCTYPE html>
+<title>@supports needs to be consistent with actual nesting support</title>
+<link rel="author" title="Steinar H. Gunderson" href="mailto:sesse@chromium.org">
+<link rel="help" href="https://crbug.com/1414012">
+<link rel="match" href="../../../../expected/wpt-import/css/css-nesting/supports-is-consistent-ref.html">
+<style>
+  /* This test is expected to pass even if the browser does not support nesting. */
+  @supports selector(&) {
+    p {
+      color: red;
+      & { color: inherit; }
+    }
+  }
+</style>
+<body>
+  <p>Test passes if this text is not red</p>
+</body>
diff --git a/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/supports-rule.html b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/supports-rule.html
new file mode 100644
index 000000000000..4d33f11acf9c
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/wpt-import/css/css-nesting/supports-rule.html
@@ -0,0 +1,55 @@
+<!DOCTYPE html>
+<title>@supports with nesting</title>
+<link rel="author" title="Matthieu Dubet" href="mailto:m_dubet@apple.com">
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/">
+<link rel="match" href="../../../../expected/wpt-import/css/css-nesting/supports-rule-ref.html">
+<style>
+  .test {
+    background-color: red;
+    width: 100px;
+    height: 100px;
+    display: grid;
+  }
+
+  @supports(not selector(> .test-1)) {
+    .test-1 {
+      background-color: green;
+    }
+  }
+
+  .test {
+    > .test-2 {
+      background-color: green;
+    }
+    @supports (selector(> .test-2)) {
+      > .test-2 {
+        background-color: red;
+      }
+    }
+  }
+
+  .test-3 {
+    @supports (selector(&)) {
+      & {
+        background-color: green;
+      }
+    }
+  }
+
+  @supports(selector(&)) {
+    .test-4 {
+      background-color: green;
+    }
+  }
+
+  body * + * {
+    margin-top: 8px;
+  }
+</style>
+<body>
+  <p>Tests pass if <strong>block is green</strong></p>
+  <div class="test test-1"></div>
+  <div class="test"><div class="test-2"></div></div>
+  <div class="test test-3"></div>
+  <div class="test test-4"></div>
+</body>
diff --git a/Tests/LibWeb/TestConfig.ini b/Tests/LibWeb/TestConfig.ini
index 405eb45eb591..c805811ad66e 100644
--- a/Tests/LibWeb/TestConfig.ini
+++ b/Tests/LibWeb/TestConfig.ini
@@ -55,3 +55,18 @@ Text/input/wpt-import/html/infrastructure/safe-passing-of-structured-data/resour
 
 ; Flaky, apparently due to font loading
 Text/input/wpt-import/css/css-flexbox/flex-item-compressible-001.html
+
+; WPT ref-tests that currently fail
+Ref/input/wpt-import/css/css-nesting/has-nesting.html
+Ref/input/wpt-import/css/css-nesting/host-nesting-003.html
+Ref/input/wpt-import/css/css-nesting/host-nesting-004.html
+Ref/input/wpt-import/css/css-nesting/nest-containing-forgiving.html
+
+; WPT crash tests are not supported yet - and probably should go in a separate directory
+Text/input/wpt-import/css/css-nesting/delete-other-rule-crash.html
+Text/input/wpt-import/css/css-nesting/implicit-parent-insertion-crash.html
+Text/input/wpt-import/css/css-nesting/pseudo-part-crash.html
+Text/input/wpt-import/css/css-nesting/pseudo-where-crash.html
+
+; Currently crashes
+Text/input/wpt-import/css/css-nesting/top-level-is-scope.html
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/block-skipping.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/block-skipping.txt
new file mode 100644
index 000000000000..bf2217da77c4
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/block-skipping.txt
@@ -0,0 +1,11 @@
+Summary
+
+Harness status: OK
+
+Rerun
+
+Found 1 tests
+
+1 Pass
+Details
+Result	Test Name	MessagePass	CSS Nesting bug: Block ignored after lack of whitespace	
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/implicit-nesting-ident-recovery.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/implicit-nesting-ident-recovery.txt
new file mode 100644
index 000000000000..c62089b82b7e
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/implicit-nesting-ident-recovery.txt
@@ -0,0 +1,12 @@
+Summary
+
+Harness status: OK
+
+Rerun
+
+Found 2 tests
+
+2 Pass
+Details
+Result	Test Name	MessagePass	Unknown declaration does not consume subsequent declaration	
+Pass	Unknown declaration with blocks does not consume subsequent declaration	
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/implicit-nesting-ident.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/implicit-nesting-ident.txt
new file mode 100644
index 000000000000..9f801e41dc20
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/implicit-nesting-ident.txt
@@ -0,0 +1,11 @@
+Summary
+
+Harness status: OK
+
+Rerun
+
+Found 1 tests
+
+1 Pass
+Details
+Result	Test Name	MessagePass	Nested rule starting with tag	
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/invalid-inner-rules.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/invalid-inner-rules.txt
new file mode 100644
index 000000000000..cf937029dbaf
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/invalid-inner-rules.txt
@@ -0,0 +1,12 @@
+Summary
+
+Harness status: OK
+
+Rerun
+
+Found 2 tests
+
+2 Fail
+Details
+Result	Test Name	MessageFail	Simple CSSOM manipulation of subrules	
+Fail	Simple CSSOM manipulation of subrules 1	
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/invalidation-001.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/invalidation-001.txt
new file mode 100644
index 000000000000..cdc3188ae136
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/invalidation-001.txt
@@ -0,0 +1,11 @@
+Summary
+
+Harness status: OK
+
+Rerun
+
+Found 1 tests
+
+1 Pass
+Details
+Result	Test Name	MessagePass	CSS Selectors nested invalidation on changed parent	
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/invalidation-002.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/invalidation-002.txt
new file mode 100644
index 000000000000..cfbdc42d7ded
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/invalidation-002.txt
@@ -0,0 +1,11 @@
+Summary
+
+Harness status: OK
+
+Rerun
+
+Found 1 tests
+
+1 Pass
+Details
+Result	Test Name	MessagePass	CSS Selectors nested invalidation on changed child	
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/invalidation-003.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/invalidation-003.txt
new file mode 100644
index 000000000000..deafe1e66522
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/invalidation-003.txt
@@ -0,0 +1,11 @@
+Summary
+
+Harness status: OK
+
+Rerun
+
+Found 1 tests
+
+1 Pass
+Details
+Result	Test Name	MessagePass	CSS Selectors nested invalidation with :has()	
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/invalidation-004.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/invalidation-004.txt
new file mode 100644
index 000000000000..6a6420886384
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/invalidation-004.txt
@@ -0,0 +1,11 @@
+Summary
+
+Harness status: OK
+
+Rerun
+
+Found 1 tests
+
+1 Pass
+Details
+Result	Test Name	MessagePass	CSS Selectors nested invalidation through @media by selectorText	
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/nested-declarations-cssom-whitespace.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/nested-declarations-cssom-whitespace.txt
new file mode 100644
index 000000000000..c3676da0108e
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/nested-declarations-cssom-whitespace.txt
@@ -0,0 +1,12 @@
+Summary
+
+Harness status: OK
+
+Rerun
+
+Found 2 tests
+
+2 Fail
+Details
+Result	Test Name	MessageFail	Empty CSSNestedDeclarations do not affect outer serialization	
+Fail	Empty CSSNestedDeclarations do not affect outer serialization (nested grouping rule)	
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/nested-declarations-cssom.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/nested-declarations-cssom.txt
new file mode 100644
index 000000000000..ce0d62b40ab4
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/nested-declarations-cssom.txt
@@ -0,0 +1,23 @@
+Summary
+
+Harness status: OK
+
+Rerun
+
+Found 12 tests
+
+6 Pass
+6 Fail
+Details
+Result	Test Name	MessageFail	Trailing declarations	
+Fail	Mixed declarations	
+Fail	CSSNestedDeclarations.style	
+Pass	Nested group rule	
+Pass	Nested @scope rule	
+Fail	Inner rule starting with an ident	
+Fail	Inserting a CSSNestedDeclaration rule into style rule	Unable to parse CSS rule.
+Fail	Inserting a CSSNestedDeclaration rule into nested group rule	Unable to parse CSS rule.
+Pass	Attempting to insert a CSSNestedDeclaration rule into top-level @media rule	
+Pass	Attempting to insert a CSSNestedDeclaration rule into a stylesheet	
+Pass	Attempting to insert a CSSNestedDeclaration rule, empty block	
+Pass	Attempting to insert a CSSNestedDeclaration rule, all invalid declarations	
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/nested-declarations-matching.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/nested-declarations-matching.txt
new file mode 100644
index 000000000000..17b3bb317f29
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/nested-declarations-matching.txt
@@ -0,0 +1,21 @@
+Summary
+
+Harness status: OK
+
+Rerun
+
+Found 11 tests
+
+11 Fail
+Details
+Result	Test Name	MessageFail	Trailing declarations apply after any preceding rules	
+Fail	Trailing declarations apply after any preceding rules (no leading)	
+Fail	Trailing declarations apply after any preceding rules (multiple)	
+Fail	Nested declarations rule has same specificity as outer selector	
+Fail	Nested declarations rule has top-level specificity behavior	
+Fail	Nested declarations rule has top-level specificity behavior (max matching)	
+Fail	Bare declartaion in nested grouping rule can match pseudo-element	
+Fail	Nested group rules have top-level specificity behavior	
+Fail	Nested @scope rules behave like :where(:scope)	
+Fail	Nested @scope rules behave like :where(:scope) (trailing)	
+Fail	Nested declarations rule responds to parent selector text change	
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/nested-rule-cssom-invalidation.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/nested-rule-cssom-invalidation.txt
new file mode 100644
index 000000000000..0b119b35299e
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/nested-rule-cssom-invalidation.txt
@@ -0,0 +1,11 @@
+Summary
+
+Harness status: OK
+
+Rerun
+
+Found 1 tests
+
+1 Fail
+Details
+Result	Test Name	MessageFail	Nested rule responds to parent selector text change	
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/nesting-layer.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/nesting-layer.txt
new file mode 100644
index 000000000000..83300023f570
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/nesting-layer.txt
@@ -0,0 +1,11 @@
+Summary
+
+Harness status: OK
+
+Rerun
+
+Found 1 tests
+
+1 Pass
+Details
+Result	Test Name	MessagePass	@layer can be nested	
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/parsing.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/parsing.txt
new file mode 100644
index 000000000000..2a3d0a1e70fc
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/parsing.txt
@@ -0,0 +1,43 @@
+Summary
+
+Harness status: OK
+
+Rerun
+
+Found 32 tests
+
+31 Pass
+1 Fail
+Details
+Result	Test Name	MessagePass	.foo { & { color: green; }}	
+Pass	.foo { &.bar { color: green; }}	
+Pass	.foo { & .bar { color: green; }}	
+Pass	.foo { & > .bar { color: green; }}	
+Pass	.foo { > .bar { color: green; }}	
+Pass	.foo { > & .bar { color: green; }}	
+Pass	.foo { + .bar & { color: green; }}	
+Pass	.foo { + .bar, .foo, > .baz { color: green; }}	
+Pass	.foo { .foo { color: green; }}	
+Pass	.foo { .test > & .bar { color: green; }}	
+Pass	.foo { .foo, .foo & { color: green; }}	
+Pass	.foo { .foo, .bar { color: green; }}	
+Pass	.foo { :is(.bar, .baz) { color: green; }}	
+Pass	.foo { &:is(.bar, .baz) { color: green; }}	
+Pass	.foo { :is(.bar, &.baz) { color: green; }}	
+Pass	.foo { &:is(.bar, &.baz) { color: green; }}	
+Pass	.foo { div& { color: green; }}	
+Fail	INVALID: .foo { &div { color: green; }}	
+Pass	.foo { .class& { color: green; }}	
+Pass	.foo { &.class { color: green; }}	
+Pass	.foo { [attr]& { color: green; }}	
+Pass	.foo { &[attr] { color: green; }}	
+Pass	.foo { #id& { color: green; }}	
+Pass	.foo { &#id { color: green; }}	
+Pass	.foo { :hover& { color: green; }}	
+Pass	.foo { &:hover { color: green; }}	
+Pass	.foo { :is(div)& { color: green; }}	
+Pass	.foo { &:is(div) { color: green; }}	
+Pass	.foo { & .bar & .baz & .qux { color: green; }}	
+Pass	.foo { && { color: green; }}	
+Pass	.foo { & > section, & > article { color: green; }}	
+Pass	.foo, .bar { & + .baz, &.qux { color: green; }}	
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/serialize-group-rules-with-decls.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/serialize-group-rules-with-decls.txt
new file mode 100644
index 000000000000..6f17ed4a4416
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/serialize-group-rules-with-decls.txt
@@ -0,0 +1,26 @@
+Summary
+
+Harness status: OK
+
+Rerun
+
+Found 15 tests
+
+4 Pass
+11 Fail
+Details
+Result	Test Name	MessageFail	Declarations are serialized on one line, rules on two.	
+Fail	Mixed declarations/rules are on two lines.	
+Fail	Implicit rule is serialized	
+Fail	Implicit rule not removed	
+Fail	Implicit + empty hover rule	
+Fail	Implicit like rule not in first position	
+Fail	Two implicit-like rules	
+Fail	Implicit like rule after decls	
+Fail	Implicit like rule after decls, missing closing braces	
+Fail	Implicit like rule with other selectors	
+Fail	Implicit-like rule in style rule	
+Pass	Empty conditional rule	
+Pass	Empty style rule	
+Pass	Empty conditional inside style rule	
+Pass	Empty style inside conditional	
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/top-level-parent-pseudo-specificity.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/top-level-parent-pseudo-specificity.txt
new file mode 100644
index 000000000000..0682c632752f
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/top-level-parent-pseudo-specificity.txt
@@ -0,0 +1,11 @@
+Summary
+
+Harness status: OK
+
+Rerun
+
+Found 1 tests
+
+1 Fail
+Details
+Result	Test Name	MessageFail	CSS Nesting: Specificity of top-level '&'	
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/input/wpt-import/common/gc.js b/Tests/LibWeb/Text/input/wpt-import/common/gc.js
new file mode 100644
index 000000000000..ac43a4cfaf77
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/common/gc.js
@@ -0,0 +1,52 @@
+/**
+ * Does a best-effort attempt at invoking garbage collection. Attempts to use
+ * the standardized `TestUtils.gc()` function, but falls back to other
+ * environment-specific nonstandard functions, with a final result of just
+ * creating a lot of garbage (in which case you will get a console warning).
+ *
+ * This should generally only be used to attempt to trigger bugs and crashes
+ * inside tests, i.e. cases where if garbage collection happened, then this
+ * should not trigger some misbehavior. You cannot rely on garbage collection
+ * successfully trigger, or that any particular unreachable object will be
+ * collected.
+ *
+ * @returns {Promise<undefined>} A promise you should await to ensure garbage
+ * collection has had a chance to complete.
+ */
+self.garbageCollect = async () => {
+  // https://testutils.spec.whatwg.org/#the-testutils-namespace
+  if (self.TestUtils?.gc) {
+    return TestUtils.gc();
+  }
+
+  // Use --expose_gc for V8 (and Node.js)
+  // to pass this flag at chrome launch use: --js-flags="--expose-gc"
+  // Exposed in SpiderMonkey shell as well
+  if (self.gc) {
+    return self.gc();
+  }
+
+  // Present in some WebKit development environments
+  if (self.GCController) {
+    return GCController.collect();
+  }
+
+  console.warn(
+    'Tests are running without the ability to do manual garbage collection. ' +
+    'They will still work, but coverage will be suboptimal.');
+
+  for (var i = 0; i < 1000; i++) {
+    gcRec(10);
+  }
+
+  function gcRec(n) {
+    if (n < 1) {
+      return {};
+    }
+
+    let temp = { i: "ab" + i + i / 100000 };
+    temp += "foo";
+
+    gcRec(n - 1);
+  }
+};
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/block-skipping.css b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/block-skipping.css
new file mode 100644
index 000000000000..c92e2dc9f795
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/block-skipping.css
@@ -0,0 +1,6 @@
+.test{span{color:red}span{color:green}}
+
+/*
+ * There needs to be some text here, or the bug
+ * will not manifest for unrelated reasons.
+ */
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/block-skipping.html b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/block-skipping.html
new file mode 100644
index 000000000000..d857e47d4dfd
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/block-skipping.html
@@ -0,0 +1,17 @@
+<!doctype html>
+<title>CSS Nesting bug: Block ignored after lack of whitespace</title>
+<link rel="help" href="https://crbug.com/362674384">
+<link rel="stylesheet" href="block-skipping.css">  <!-- Bug only manifests in external stylesheets. -->
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+
+<div class="test">
+  <span id="target">Test passes if this text is green.</span>
+</div>
+
+<script>
+test(() => {
+  assert_equals(getComputedStyle(target).color, 'rgb(0, 128, 0)');
+});
+</script>
+
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/delete-other-rule-crash.html b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/delete-other-rule-crash.html
new file mode 100644
index 000000000000..a8319ce25b0c
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/delete-other-rule-crash.html
@@ -0,0 +1,15 @@
+<!DOCTYPE html>
+<body>
+<title>Crash with lazy parsing child rules and stylesheet copy-on-write</title>
+<link rel="help" href="https://crbug.com/1404879">
+<link href="../support/delete-other-rule-crash.css" rel="stylesheet">
+<script src="../../common/gc.js"></script>
+<script>
+addEventListener('DOMContentLoaded', async () => {
+  requestAnimationFrame(async () => {
+    document.styleSheets[0].deleteRule(0);
+    await garbageCollect();
+    document.styleSheets[0].cssRules[0].cssText;
+  });
+});
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/implicit-nesting-ident-recovery.html b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/implicit-nesting-ident-recovery.html
new file mode 100644
index 000000000000..2329a16c3259
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/implicit-nesting-ident-recovery.html
@@ -0,0 +1,29 @@
+<!DOCTYPE html>
+<title>CSS Nesting: Nesting, error recovery</title>
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/">
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+<style>
+  #target1 {
+    display:block;
+    display:new-block;
+    color:green;
+  }
+  #target2 {
+    display:block;
+    display:hover {};
+    color:green;
+  }
+</style>
+<div id=target1>Green</div>
+<div id=target2>Green</div>
+<script>
+  test(() => {
+    assert_equals(getComputedStyle(target1).color, 'rgb(0, 128, 0)');
+  }, 'Unknown declaration does not consume subsequent declaration');
+</script>
+<script>
+  test(() => {
+    assert_equals(getComputedStyle(target2).color, 'rgb(0, 128, 0)');
+  }, 'Unknown declaration with blocks does not consume subsequent declaration');
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/implicit-nesting-ident.html b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/implicit-nesting-ident.html
new file mode 100644
index 000000000000..e4049fb8fd3c
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/implicit-nesting-ident.html
@@ -0,0 +1,22 @@
+<!DOCTYPE html>
+<title>CSS Nesting: Implicit nesting (ident)</title>
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/">
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+<style>
+  #main {
+    div {
+      color: green;
+    }
+  }
+</style>
+<div id=main>
+  <div id=target>
+    Green
+  </div>
+</main>
+<script>
+  test(() => {
+    assert_equals(getComputedStyle(target).color, 'rgb(0, 128, 0)');
+  }, 'Nested rule starting with tag');
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/implicit-parent-insertion-crash.html b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/implicit-parent-insertion-crash.html
new file mode 100644
index 000000000000..4be1e1c8f9ed
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/implicit-parent-insertion-crash.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<body>
+<title>Use-after-free when inserting implicit parent selector</title>
+<link rel="help" href="https://crbug.com/1380313">
+<style>
+:root {
+  :lang(en), :lang(en) {
+  }
+}
+</style>
+<div lang="en"></div>
+<script>
+  // Allocate a large chunk of memory, to trigger a GC.
+  new Int32Array(536870911);
+</script>
+
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/invalid-inner-rules.html b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/invalid-inner-rules.html
new file mode 100644
index 000000000000..9551583d0af3
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/invalid-inner-rules.html
@@ -0,0 +1,56 @@
+<!doctype html>
+<title>Simple CSSOM manipulation of subrules</title>
+<link rel="author" title="Steinar H. Gunderson" href="mailto:sesse@chromium.org">
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/">
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+
+<style id="ss">
+div {
+  /* This is not a conditional rule, and thus cannot be in nesting context. */
+  @font-face {
+    &.a { font-size: 10px; }
+  }
+
+  @media screen {
+    &.a { color: red; }
+
+    /* Same. */
+    @font-face {
+      &.a { font-size: 10px; }
+    }
+  }
+}
+</style>
+
+<script>
+  test(() => {
+    let [ss] = document.styleSheets;
+    assert_equals(ss.cssRules.length, 1);
+
+    // The @font-face rule should be ignored.
+    assert_equals(ss.cssRules[0].cssText,
+`div {
+  @media screen {
+  &.a { color: red; }
+}
+}`);
+  });
+
+  test(() => {
+    let [ss] = document.styleSheets;
+    assert_equals(ss.cssRules.length, 1);
+    assert_throws_dom('HierarchyRequestError',
+      () => { ss.cssRules[0].cssRules[0].insertRule('@font-face {}', 0); });
+    assert_throws_dom('HierarchyRequestError',
+      () => { ss.cssRules[0].insertRule('@font-face {}', 0); });
+
+    // The @font-face rules should be ignored (again).
+    assert_equals(ss.cssRules[0].cssText,
+`div {
+  @media screen {
+  &.a { color: red; }
+}
+}`);
+  });
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/invalidation-001.html b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/invalidation-001.html
new file mode 100644
index 000000000000..42094ffc698b
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/invalidation-001.html
@@ -0,0 +1,33 @@
+<!doctype html>
+<title>CSS Selectors nested invalidation on changed parent</title>
+<link rel="author" title="Steinar H. Gunderson" href="mailto:sesse@chromium.org">
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/">
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+
+<style>
+  .b {
+    color: red;
+  }
+  .a {
+    & .b {
+      color: green;
+    }
+  }
+</style>
+
+<div id="container">
+  <div id="child" class="b">
+    Test passes if color is green.
+  </div>
+</div>
+
+<script>
+  test(() => {
+    let container = document.getElementById('container');
+    let child = document.getElementById('child');
+    assert_equals(getComputedStyle(child).color, 'rgb(255, 0, 0)');
+    container.classList.add('a');
+    assert_equals(getComputedStyle(child).color, 'rgb(0, 128, 0)');
+  });
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/invalidation-002.html b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/invalidation-002.html
new file mode 100644
index 000000000000..a2a4564c8e26
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/invalidation-002.html
@@ -0,0 +1,33 @@
+<!doctype html>
+<title>CSS Selectors nested invalidation on changed child</title>
+<link rel="author" title="Steinar H. Gunderson" href="mailto:sesse@chromium.org">
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/">
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+
+<style>
+  .a {
+    color: green;
+  }
+  .a {
+    & .b {
+      color: red;
+    }
+  }
+</style>
+
+<div id="container" class="a">
+  <div id="child" class="b">
+    Test passes if color is green.
+  </div>
+</div>
+
+<script>
+  test(() => {
+    let container = document.getElementById('container');
+    let child = document.getElementById('child');
+    assert_equals(getComputedStyle(child).color, 'rgb(255, 0, 0)');
+    child.classList.remove('b');
+    assert_equals(getComputedStyle(child).color, 'rgb(0, 128, 0)');
+  });
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/invalidation-003.html b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/invalidation-003.html
new file mode 100644
index 000000000000..1f80bab004d2
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/invalidation-003.html
@@ -0,0 +1,34 @@
+<!doctype html>
+<title>CSS Selectors nested invalidation with :has()</title>
+<link rel="author" title="Steinar H. Gunderson" href="mailto:sesse@chromium.org">
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/">
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+
+<style>
+  .a {
+    color: red;
+    :has(&) {
+      color: green;
+    }
+  }
+</style>
+
+<div id="container">
+  Test passes if color is green.
+  <div>
+    <div id="child"></div>
+  </div>
+</div>
+
+<script>
+  test(() => {
+    let container = document.getElementById('container');
+    let child = document.getElementById('child');
+    assert_equals(getComputedStyle(container).color, 'rgb(0, 0, 0)');
+    assert_equals(getComputedStyle(child).color, 'rgb(0, 0, 0)');
+    child.classList.add('a');
+    assert_equals(getComputedStyle(container).color, 'rgb(0, 128, 0)');
+    assert_equals(getComputedStyle(child).color, 'rgb(255, 0, 0)');
+  });
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/invalidation-004.html b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/invalidation-004.html
new file mode 100644
index 000000000000..de982984554c
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/invalidation-004.html
@@ -0,0 +1,30 @@
+<!doctype html>
+<title>CSS Selectors nested invalidation through @media by selectorText</title>
+<link rel="author" title="Steinar H. Gunderson" href="mailto:sesse@chromium.org">
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/">
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+
+<style>
+  .b {
+    color: red;
+  }
+  & {
+    @media screen {
+      &.b { color: green; }
+    }
+  }
+</style>
+
+<div id="elem" class="a b">
+  Test passes if color is green.
+</div>
+
+<script>
+  test(() => {
+    let elem = document.getElementById('elem');
+    assert_equals(getComputedStyle(elem).color, 'rgb(255, 0, 0)');
+    document.styleSheets[0].rules[1].selectorText = '.a';
+    assert_equals(getComputedStyle(elem).color, 'rgb(0, 128, 0)');
+  });
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/nested-declarations-cssom-whitespace.html b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/nested-declarations-cssom-whitespace.html
new file mode 100644
index 000000000000..280732d8709d
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/nested-declarations-cssom-whitespace.html
@@ -0,0 +1,52 @@
+<!DOCTYPE html>
+<title>CSS Nesting: CSSNestedDeclarations CSSOM (whitespace)</title>
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/#nested-declarations-rule">
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+<script>
+  // https://drafts.csswg.org/cssom/#serialize-a-css-rule
+  test(() => {
+    let s = new CSSStyleSheet();
+    s.replaceSync(`
+      .a {
+        & { }
+        left: 1px;
+        & { }
+        right: 1px;
+      }
+    `);
+    assert_equals(s.cssRules.length, 1);
+    let a_rule = s.cssRules[0];
+    assert_equals(a_rule.cssRules.length, 4);
+    for (let child_rule of a_rule.cssRules) {
+      child_rule.style = '';
+    }
+    assert_equals(a_rule.cssText, '.a {\n  & { }\n  & { }\n}');
+  }, 'Empty CSSNestedDeclarations do not affect outer serialization');
+
+  // https://drafts.csswg.org/cssom/#serialize-a-css-rule
+  test(() => {
+    let s = new CSSStyleSheet();
+    s.replaceSync(`
+      .a {
+        @media (width > 1px) {
+          & { }
+          left: 1px;
+          & { }
+          right: 1px;
+        }
+      }
+    `);
+    assert_equals(s.cssRules.length, 1);
+    let outer = s.cssRules[0];
+    assert_equals(outer.cssRules.length, 1);
+
+    // @media
+    let media = outer.cssRules[0];
+    assert_equals(media.cssRules.length, 4);
+    for (let child_rule of media.cssRules) {
+      child_rule.style = '';
+    }
+    assert_equals(media.cssText, '@media (width > 1px) {\n  & { }\n  & { }\n}');
+  }, 'Empty CSSNestedDeclarations do not affect outer serialization (nested grouping rule)');
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/nested-declarations-cssom.html b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/nested-declarations-cssom.html
new file mode 100644
index 000000000000..2f396714c564
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/nested-declarations-cssom.html
@@ -0,0 +1,237 @@
+<!DOCTYPE html>
+<title>CSS Nesting: CSSNestedDeclarations CSSOM</title>
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/#nested-declarations-rule">
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+<script>
+  test(() => {
+    let s = new CSSStyleSheet();
+    s.replaceSync(`
+      .a {
+        & { --x:1; }
+        --x:2;
+      }
+    `);
+    assert_equals(s.cssRules.length, 1);
+    let outer = s.cssRules[0];
+    assert_equals(outer.cssRules.length, 2);
+    assert_equals(outer.cssRules[0].cssText, `& { --x: 1; }`);
+    assert_equals(outer.cssRules[1].cssText, `--x: 2;`);
+  }, 'Trailing declarations');
+
+  test(() => {
+    let s = new CSSStyleSheet();
+    s.replaceSync(`
+      .a {
+        --a:1;
+        --b:1;
+        & { --c:1; }
+        --d:1;
+        --e:1;
+        & { --f:1; }
+        --g:1;
+        --h:1;
+        --i:1;
+        & { --j:1; }
+        --k:1;
+        --l:1;
+      }
+    `);
+    assert_equals(s.cssRules.length, 1);
+    let outer = s.cssRules[0];
+    assert_equals(outer.cssRules.length, 6);
+    assert_equals(outer.cssRules[0].cssText, `& { --c: 1; }`);
+    assert_equals(outer.cssRules[1].cssText, `--d: 1; --e: 1;`);
+    assert_equals(outer.cssRules[2].cssText, `& { --f: 1; }`);
+    assert_equals(outer.cssRules[3].cssText, `--g: 1; --h: 1; --i: 1;`);
+    assert_equals(outer.cssRules[4].cssText, `& { --j: 1; }`);
+    assert_equals(outer.cssRules[5].cssText, `--k: 1; --l: 1;`);
+  }, 'Mixed declarations');
+
+  test(() => {
+    let s = new CSSStyleSheet();
+    s.replaceSync(`
+      .a {
+        & { --x:1; }
+        --y:2;
+        --z:3;
+      }
+    `);
+    assert_equals(s.cssRules.length, 1);
+    let outer = s.cssRules[0];
+    assert_equals(outer.cssRules.length, 2);
+    let nested_declarations = outer.cssRules[1];
+    assert_true(nested_declarations instanceof CSSNestedDeclarations);
+    assert_equals(nested_declarations.style.length, 2);
+    assert_equals(nested_declarations.style.getPropertyValue('--x'), '');
+    assert_equals(nested_declarations.style.getPropertyValue('--y'), '2');
+    assert_equals(nested_declarations.style.getPropertyValue('--z'), '3');
+  }, 'CSSNestedDeclarations.style');
+
+  test(() => {
+    let s = new CSSStyleSheet();
+    s.replaceSync(`
+      .a {
+        @media (width > 100px) {
+          --x:1;
+          --y:1;
+          .b { }
+          --z:1;
+        }
+        --w:1;
+      }
+    `);
+    assert_equals(s.cssRules.length, 1);
+    let outer = s.cssRules[0];
+    assert_equals(outer.cssRules.length, 2);
+
+    // @media
+    let media = outer.cssRules[0];
+    assert_equals(media.cssRules.length, 3);
+    assert_true(media.cssRules[0] instanceof CSSNestedDeclarations);
+    assert_equals(media.cssRules[0].cssText, `--x: 1; --y: 1;`);
+    assert_equals(media.cssRules[1].cssText, `& .b { }`);
+    assert_true(media.cssRules[2] instanceof CSSNestedDeclarations);
+    assert_equals(media.cssRules[2].cssText, `--z: 1;`);
+
+    assert_true(outer.cssRules[1] instanceof CSSNestedDeclarations);
+    assert_equals(outer.cssRules[1].cssText, `--w: 1;`);
+  }, 'Nested group rule');
+
+  test(() => {
+    let s = new CSSStyleSheet();
+    s.replaceSync(`
+      .a {
+        @scope (.foo) {
+          --x:1;
+          --y:1;
+          .b { }
+          --z:1;
+        }
+        --w:1;
+      }
+    `);
+    assert_equals(s.cssRules.length, 1);
+    let outer = s.cssRules[0];
+    if (window.CSSScopeRule) {
+      assert_equals(outer.cssRules.length, 2);
+
+      // @scope
+      let scope = outer.cssRules[0];
+      assert_true(scope instanceof CSSScopeRule);
+      assert_equals(scope.cssRules.length, 3);
+      assert_true(scope.cssRules[0] instanceof CSSNestedDeclarations);
+      assert_equals(scope.cssRules[0].cssText, `--x: 1; --y: 1;`);
+      assert_equals(scope.cssRules[1].cssText, `.b { }`); // Implicit :scope here.
+      assert_true(scope.cssRules[2] instanceof CSSNestedDeclarations);
+      assert_equals(scope.cssRules[2].cssText, `--z: 1;`);
+
+      assert_true(outer.cssRules[1] instanceof CSSNestedDeclarations);
+      assert_equals(outer.cssRules[1].cssText, `--w: 1;`);
+    } else {
+      assert_equals(outer.cssRules.length, 0);
+    }
+  }, 'Nested @scope rule');
+
+  test(() => {
+    let s = new CSSStyleSheet();
+    s.replaceSync(`
+      a {
+        & { --x:1; }
+        width: 100px;
+        height: 200px;
+        color:hover {}
+        --y: 2;
+      }
+    `);
+    assert_equals(s.cssRules.length, 1);
+    let outer = s.cssRules[0];
+    assert_equals(outer.cssRules.length, 4);
+    assert_equals(outer.cssRules[0].cssText, `& { --x: 1; }`);
+    assert_equals(outer.cssRules[1].cssText, `width: 100px; height: 200px;`);
+    assert_equals(outer.cssRules[2].cssText, `& color:hover { }`);
+    assert_equals(outer.cssRules[3].cssText, `--y: 2;`);
+  }, 'Inner rule starting with an ident');
+
+  test(() => {
+    let s = new CSSStyleSheet();
+    s.replaceSync('.a {}');
+    assert_equals(s.cssRules.length, 1);
+    let a_rule = s.cssRules[0];
+    assert_equals(a_rule.cssRules.length, 0);
+    a_rule.insertRule(`
+      width: 100px;
+      height: 200px;
+    `);
+    assert_equals(a_rule.cssRules.length, 1);
+    assert_true(a_rule.cssRules[0] instanceof CSSNestedDeclarations);
+    assert_equals(a_rule.cssRules[0].cssText, `width: 100px; height: 200px;`);
+  }, 'Inserting a CSSNestedDeclaration rule into style rule');
+
+  test(() => {
+    let s = new CSSStyleSheet();
+    s.replaceSync('.a { @media (width > 100px) {} }');
+    assert_equals(s.cssRules.length, 1);
+    assert_equals(s.cssRules[0].cssRules.length, 1);
+    let media_rule = s.cssRules[0].cssRules[0];
+    assert_true(media_rule instanceof CSSMediaRule);
+    assert_equals(media_rule.cssRules.length, 0);
+    media_rule.insertRule(`
+      width: 100px;
+      height: 200px;
+    `);
+    assert_equals(media_rule.cssRules.length, 1);
+    assert_true(media_rule.cssRules[0] instanceof CSSNestedDeclarations);
+    assert_equals(media_rule.cssRules[0].cssText, `width: 100px; height: 200px;`);
+  }, 'Inserting a CSSNestedDeclaration rule into nested group rule');
+
+  test(() => {
+    let s = new CSSStyleSheet();
+    s.replaceSync('@media (width > 100px) {}');
+    assert_equals(s.cssRules.length, 1);
+    let media_rule = s.cssRules[0];
+    assert_true(media_rule instanceof CSSMediaRule);
+    assert_equals(media_rule.cssRules.length, 0);
+    assert_throws_dom('SyntaxError', () => {
+      media_rule.insertRule(`
+        width: 100px;
+        height: 200px;
+      `);
+    });
+  }, 'Attempting to insert a CSSNestedDeclaration rule into top-level @media rule');
+
+  test(() => {
+    let sheet = new CSSStyleSheet();
+    assert_throws_dom('SyntaxError', () => {
+      sheet.insertRule(`
+        width: 100px;
+        height: 200px;
+      `);
+    });
+  }, 'Attempting to insert a CSSNestedDeclaration rule into a stylesheet');
+
+  test(() => {
+    let s = new CSSStyleSheet();
+    s.replaceSync('.a {}');
+    assert_equals(s.cssRules.length, 1);
+    let a_rule = s.cssRules[0];
+    assert_equals(a_rule.cssRules.length, 0);
+    assert_throws_dom('SyntaxError', () => {
+      a_rule.insertRule('');
+    });
+  }, 'Attempting to insert a CSSNestedDeclaration rule, empty block');
+
+  test(() => {
+    let s = new CSSStyleSheet();
+    s.replaceSync('.a {}');
+    assert_equals(s.cssRules.length, 1);
+    let a_rule = s.cssRules[0];
+    assert_equals(a_rule.cssRules.length, 0);
+    assert_throws_dom('SyntaxError', () => {
+    a_rule.insertRule(`
+      xwidth: 100px;
+      xheight: 200px;
+    `);
+    });
+  }, 'Attempting to insert a CSSNestedDeclaration rule, all invalid declarations');
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/nested-declarations-matching.html b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/nested-declarations-matching.html
new file mode 100644
index 000000000000..9d68478ad9ab
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/nested-declarations-matching.html
@@ -0,0 +1,221 @@
+<!DOCTYPE html>
+<title>CSS Nesting: CSSNestedDeclarations matching</title>
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/#nested-declarations-rule">
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+
+<style>
+  .trailing {
+    --x: FAIL;
+    & { --x: FAIL; }
+    --x: PASS;
+  }
+</style>
+<div class=trailing></div>
+<script>
+  test(() => {
+    let e = document.querySelector('.trailing');
+    assert_equals(getComputedStyle(e).getPropertyValue('--x'), 'PASS');
+  }, 'Trailing declarations apply after any preceding rules');
+</script>
+
+
+<style>
+  .trailing_no_leading {
+    & { --x: FAIL; }
+    --x: PASS;
+  }
+</style>
+<div class=trailing_no_leading></div>
+<script>
+  test(() => {
+    let e = document.querySelector('.trailing_no_leading');
+    assert_equals(getComputedStyle(e).getPropertyValue('--x'), 'PASS');
+  }, 'Trailing declarations apply after any preceding rules (no leading)');
+</script>
+
+
+<style>
+  .trailing_multiple {
+    --x: FAIL;
+    --y: FAIL;
+    --z: FAIL;
+    --w: FAIL;
+    & { --x: FAIL; }
+    --x: PASS;
+    --y: PASS;
+    & { --z: FAIL; }
+    --z: PASS;
+    --w: PASS;
+  }
+</style>
+<div class=trailing_multiple></div>
+<script>
+  test(() => {
+    let e = document.querySelector('.trailing_multiple');
+    let s = getComputedStyle(e);
+    assert_equals(s.getPropertyValue('--x'), 'PASS');
+    assert_equals(s.getPropertyValue('--y'), 'PASS');
+    assert_equals(s.getPropertyValue('--z'), 'PASS');
+    assert_equals(s.getPropertyValue('--w'), 'PASS');
+  }, 'Trailing declarations apply after any preceding rules (multiple)');
+</script>
+
+
+<style>
+  .trailing_specificity {
+    --x: FAIL;
+    :is(&, div.nomatch2) { --x: PASS; } /* Specificity: (0, 1, 1) */
+    --x: FAIL; /* Specificity: (0, 1, 0) */
+  }
+</style>
+<div class=trailing_specificity></div>
+<script>
+  test(() => {
+    let e = document.querySelector('.trailing_specificity');
+    assert_equals(getComputedStyle(e).getPropertyValue('--x'), 'PASS');
+  }, 'Nested declarations rule has same specificity as outer selector');
+</script>
+
+
+<style>
+  #nomatch, .specificity_top_level {
+    --x: FAIL;
+    :is(&, div.nomatch2) { --x: PASS; } /* Specificity: (0, 1, 1) */
+    --x: FAIL; /* Specificity: (0, 1, 0). In particular, this does not have
+               specificity like :is(#nomatch, .specificity_top_level). */
+  }
+</style>
+<div class=specificity_top_level></div>
+<script>
+  test(() => {
+    let e = document.querySelector('.specificity_top_level');
+    assert_equals(getComputedStyle(e).getPropertyValue('--x'), 'PASS');
+  }, 'Nested declarations rule has top-level specificity behavior');
+</script>
+
+
+<style>
+  #nomatch, .specificity_top_level_max, div.specificity_top_level_max {
+    --x: FAIL;
+    :is(:where(&), div.nomatch2) { --x: FAIL; } /* Specificity: (0, 1, 1) */
+    --x: PASS; /* Specificity: (0, 1, 1) (for div.specificity_top_level_max) */
+  }
+</style>
+<div class=specificity_top_level_max></div>
+<script>
+  test(() => {
+    let e = document.querySelector('.specificity_top_level_max');
+    assert_equals(getComputedStyle(e).getPropertyValue('--x'), 'PASS');
+  }, 'Nested declarations rule has top-level specificity behavior (max matching)');
+</script>
+
+<style>
+  .nested_pseudo::after {
+    --x: FAIL;
+    @media (width > 0px) {
+      --x: PASS;
+    }
+  }
+</style>
+<div class=nested_pseudo></div>
+<script>
+  test(() => {
+    let e = document.querySelector('.nested_pseudo');
+    assert_equals(getComputedStyle(e, '::after').getPropertyValue('--x'), 'PASS');
+  }, 'Bare declartaion in nested grouping rule can match pseudo-element');
+</script>
+
+<style>
+  #nomatch, .nested_group_rule {
+    --x: FAIL;
+    @media (width > 0px) {
+      --x: FAIL; /* Specificity: (0, 1, 0) */
+    }
+    --x: PASS;
+  }
+</style>
+<div class=nested_group_rule></div>
+<script>
+  test(() => {
+    let e = document.querySelector('.nested_group_rule');
+    assert_equals(getComputedStyle(e).getPropertyValue('--x'), 'PASS');
+  }, 'Nested group rules have top-level specificity behavior');
+</script>
+
+
+<style>
+  .nested_scope_rule {
+    div:where(&) { /* Specificity: (0, 0, 1) */
+      --x: PASS;
+    }
+    @scope (&) {
+      --x: FAIL; /* Specificity: (0, 0, 0) */
+    }
+  }
+</style>
+<div class=nested_scope_rule></div>
+<script>
+  test(() => {
+    let e = document.querySelector('.nested_scope_rule');
+    assert_equals(getComputedStyle(e).getPropertyValue('--x'), 'PASS');
+  }, 'Nested @scope rules behave like :where(:scope)');
+</script>
+
+
+<style>
+  .nested_scope_rule_trailing {
+    div:where(&) { /* Specificity: (0, 0, 1) */
+      --x: PASS;
+    }
+    @scope (&) {
+      --ignored: 1;
+      .ignored {}
+      --x: FAIL; /* Specificity: (0, 0, 0) */
+    }
+  }
+</style>
+<div class=nested_scope_rule_trailing></div>
+<script>
+  test(() => {
+    let e = document.querySelector('.nested_scope_rule_trailing');
+    assert_equals(getComputedStyle(e).getPropertyValue('--x'), 'PASS');
+  }, 'Nested @scope rules behave like :where(:scope) (trailing)');
+</script>
+
+
+<style id=set_parent_selector_text_style>
+  .set_parent_selector_text {
+    div {
+      color: red;
+    }
+    .a1 {
+      .ignored {}
+      color: green;
+    }
+  }
+</style>
+<div class=set_parent_selector_text>
+  <div class=a1>A1</div>
+  <div class=a2>A2</div>
+</div>
+<script>
+  test(() => {
+    let a1 = document.querySelector('.set_parent_selector_text > .a1');
+    let a2 = document.querySelector('.set_parent_selector_text > .a2');
+    assert_equals(getComputedStyle(a1).color, 'rgb(0, 128, 0)');
+    assert_equals(getComputedStyle(a2).color, 'rgb(255, 0, 0)');
+
+    let rules = set_parent_selector_text_style.sheet.cssRules;
+    assert_equals(rules.length, 1);
+    assert_equals(rules[0].cssRules.length, 2);
+
+    let a_rule = rules[0].cssRules[1];
+    assert_equals(a_rule.selectorText, '& .a1');
+    a_rule.selectorText = '.a2';
+    assert_equals(a_rule.selectorText, '& .a2');
+
+    assert_equals(getComputedStyle(a1).color, 'rgb(255, 0, 0)');
+    assert_equals(getComputedStyle(a2).color, 'rgb(0, 128, 0)');
+  }, 'Nested declarations rule responds to parent selector text change');
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/nested-rule-cssom-invalidation.html b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/nested-rule-cssom-invalidation.html
new file mode 100644
index 000000000000..798da1e8ab61
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/nested-rule-cssom-invalidation.html
@@ -0,0 +1,41 @@
+<!DOCTYPE html>
+<title>CSS Nesting: Style invalidates after CSSOM mutations to nested rules</title>
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/#nested-style-rule">
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/#nested-group-rules">
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+
+<style id=set_parent_selector_text_style>
+  .set_parent_selector_text {
+    div {
+      color: red;
+    }
+    .a1 {
+      color: green;
+    }
+  }
+</style>
+<div class=set_parent_selector_text>
+  <div class=a1>A1</div>
+  <div class=a2>A2</div>
+</div>
+<script>
+  test(() => {
+    let a1 = document.querySelector('.set_parent_selector_text > .a1');
+    let a2 = document.querySelector('.set_parent_selector_text > .a2');
+    assert_equals(getComputedStyle(a1).color, 'rgb(0, 128, 0)');
+    assert_equals(getComputedStyle(a2).color, 'rgb(255, 0, 0)');
+
+    let rules = set_parent_selector_text_style.sheet.cssRules;
+    assert_equals(rules.length, 1);
+    assert_equals(rules[0].cssRules.length, 2);
+
+    let a_rule = rules[0].cssRules[1];
+    assert_equals(a_rule.selectorText, '& .a1');
+    a_rule.selectorText = '.a2';
+
+    assert_equals(getComputedStyle(a1).color, 'rgb(255, 0, 0)');
+    assert_equals(getComputedStyle(a2).color, 'rgb(0, 128, 0)');
+  }, 'Nested rule responds to parent selector text change');
+</script>
+
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/nesting-layer.html b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/nesting-layer.html
new file mode 100644
index 000000000000..110c9536f95a
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/nesting-layer.html
@@ -0,0 +1,52 @@
+<!DOCTYPE html>
+<title>Nested @layers</title>
+<link rel="help" href="https://drafts.csswg.org/css-nesting/#nested-group-rules">
+<link rel="help" href="https://drafts.csswg.org/css-cascade-5/#layering">
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+<style>
+
+  .a {
+    /* This should have no effect. Only at-rules containing style rules
+       are vaild when nested. */
+    @layer theme, base;
+  }
+
+  /* The theme layer wins over the base layer. */
+  @layer base, theme;
+
+  .a {
+    @layer theme {
+      & {
+        z-index: 1;
+      }
+
+      .b {
+        background-color: green;
+      }
+    }
+  }
+
+  @layer base {
+    .a {
+      z-index: 0;
+    }
+    .a .b {
+      background-color: red;
+    }
+  }
+</style>
+<main>
+  <div class="a">
+    <div class="b">
+    </div>
+  </div>
+</main>
+<script>
+  test(() => {
+    let a = document.querySelector("main > .a");
+    let b = document.querySelector("main > .a > .b");
+    assert_equals(getComputedStyle(a).zIndex, "1");
+    assert_equals(getComputedStyle(b).backgroundColor, "rgb(0, 128, 0)");
+  }, '@layer can be nested');
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/parsing.html b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/parsing.html
new file mode 100644
index 000000000000..aa59338522ca
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/parsing.html
@@ -0,0 +1,87 @@
+<!doctype html>
+<title>CSS Selectors parsing</title>
+<link rel="author" title="Adam Argyle" href="mailto:argyle@google.com">
+<link rel="author" title="Tab Atkins-Bittner" href="https://tabatkins.com/contact/">
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/">
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+
+<style id="test-sheet"></style>
+<script>
+  let [ss] = document.styleSheets
+
+  function resetStylesheet() {
+    while (ss.rules.length)
+      ss.removeRule(0)
+  }
+
+  function testNestedSelector(sel, {expected=sel, parent=".foo"}={}) {
+    resetStylesheet();
+    const ruleText = `${parent} { ${sel} { color: green; }}`
+    test(()=>{
+      ss.insertRule(ruleText);
+      assert_equals(ss.rules.length, 1, "Outer rule should exist.");
+      const rule = ss.rules[0];
+      assert_equals(rule.cssRules.length, 1, "Inner rule should exist.");
+      const innerRule = rule.cssRules[0];
+      assert_equals(innerRule.selectorText, expected, `Inner rule's selector should be "${expected}".`);
+    }, ruleText);
+  }
+
+  function testInvalidNestingSelector(sel, {parent=".foo"}={}) {
+    resetStylesheet();
+    const ruleText = `${parent} { ${sel} { color: green; }}`
+    test(()=>{
+      ss.insertRule(ruleText);
+      assert_equals(ss.rules.length, 1, "Outer rule should exist.");
+      const rule = ss.rules[0];
+      assert_equals(rule.cssRules.length, 0, "Inner rule should not exist.");
+    }, "INVALID: " + ruleText);
+  }
+
+  // basic usage
+  testNestedSelector("&");
+  testNestedSelector("&.bar");
+  testNestedSelector("& .bar");
+  testNestedSelector("& > .bar");
+
+  // relative selector
+  testNestedSelector("> .bar", {expected:"& > .bar"});
+  testNestedSelector("> & .bar", {expected:"& > & .bar"});
+  testNestedSelector("+ .bar &", {expected:"& + .bar &"});
+  testNestedSelector("+ .bar, .foo, > .baz", {expected:"& + .bar, & .foo, & > .baz"});
+
+  // implicit relative (and not)
+  testNestedSelector(".foo", {expected:"& .foo"});
+  testNestedSelector(".test > & .bar");
+  testNestedSelector(".foo, .foo &", {expected:"& .foo, .foo &"});
+  testNestedSelector(".foo, .bar", {expected:"& .foo, & .bar"});
+  testNestedSelector(":is(.bar, .baz)", {expected:"& :is(.bar, .baz)"});
+  testNestedSelector("&:is(.bar, .baz)");
+  testNestedSelector(":is(.bar, &.baz)");
+  testNestedSelector("&:is(.bar, &.baz)");
+
+  // Mixing nesting selector with other simple selectors
+  testNestedSelector("div&");
+  testInvalidNestingSelector("&div"); // type selector must be first
+  testNestedSelector(".class&");
+  testNestedSelector("&.class");
+  testNestedSelector("[attr]&");
+  testNestedSelector("&[attr]");
+  testNestedSelector("#id&");
+  testNestedSelector("&#id");
+  testNestedSelector(":hover&");
+  testNestedSelector("&:hover");
+  testNestedSelector(":is(div)&");
+  testNestedSelector("&:is(div)");
+
+  // Multiple nesting selectors
+  testNestedSelector("& .bar & .baz & .qux");
+  testNestedSelector("&&");
+
+  // Selector list in inner rule
+  testNestedSelector("& > section, & > article");
+
+  // Selector list in both inner and outer rule.
+  testNestedSelector("& + .baz, &.qux", {parent:".foo, .bar"});
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/pseudo-part-crash.html b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/pseudo-part-crash.html
new file mode 100644
index 000000000000..3ab521d71c84
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/pseudo-part-crash.html
@@ -0,0 +1,12 @@
+<!DOCTYPE html>
+<meta charset="UTF-8">
+<title>Nesting pseudo element selectors should not crash</title>
+<link rel="help" href="https://crbug.com/1376227">
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/7912">
+<style>
+  div::part(x) {
+    & {
+      color: red;
+    }
+  }
+</style>
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/pseudo-where-crash.html b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/pseudo-where-crash.html
new file mode 100644
index 000000000000..c069872c60be
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/pseudo-where-crash.html
@@ -0,0 +1,9 @@
+<!DOCTYPE html>
+<style>
+  .foo {
+    ::before:where(&) {
+      color: red;
+    }
+  }
+</style>
+<div class=foo></div>
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/serialize-group-rules-with-decls.html b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/serialize-group-rules-with-decls.html
new file mode 100644
index 000000000000..d8391cb5c56b
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/serialize-group-rules-with-decls.html
@@ -0,0 +1,91 @@
+<!doctype html>
+<title>Serialization of declarations in group rules</title>
+<link rel="author" title="Steinar H. Gunderson" href="mailto:sesse@chromium.org">
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/7850">
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+
+<style id="test-sheet"></style>
+<script>
+  function serialize(cssText) {
+    let [ss] = document.styleSheets;
+    while (ss.rules.length) {
+      ss.removeRule(0)
+    }
+    ss.insertRule(cssText);
+    return ss.rules[0].cssText;
+  }
+
+  function assert_unchanged(cssText, description) {
+    test(() => {
+      assert_equals(serialize(cssText), cssText, description);
+    }, description);
+  }
+
+  function assert_becomes(cssText, serializedCssText, description) {
+    test(() => {
+      assert_equals(serialize(cssText), serializedCssText, description);
+    }, description);
+  }
+
+  assert_unchanged(
+    "@media screen {\n  div { color: red; background-color: green; }\n}",
+    "Declarations are serialized on one line, rules on two."
+  )
+
+  assert_becomes(
+    "div { @media screen { color: red; background-color: green; } }",
+    "div {\n  @media screen {\n  color: red; background-color: green;\n}\n}",
+    "Mixed declarations/rules are on two lines."
+  );
+  assert_becomes(
+    "div {\n  @supports selector(&) { color: red; background-color: green; } &:hover { color: navy; } }",
+    "div {\n  @supports selector(&) {\n  color: red; background-color: green;\n}\n  &:hover { color: navy; }\n}",
+    "Implicit rule is serialized",
+  );
+
+  assert_unchanged("div {\n  @media screen {\n  & { color: red; }\n}\n}", "Implicit rule not removed");
+  assert_becomes(
+    "div { @media screen { & { color: red; &:hover { } } }",
+    "div {\n  @media screen {\n  & {\n  color: red;\n  &:hover { }\n}\n}\n}",
+    "Implicit + empty hover rule"
+  );
+  assert_becomes(
+    "div { @media screen { &.cls { color: red; } & { color: red; }",
+    "div {\n  @media screen {\n  &.cls { color: red; }\n  & { color: red; }\n}\n}",
+    "Implicit like rule not in first position"
+  );
+  assert_becomes(
+    "div { @media screen { & { color: red; } & { color: red; }",
+    "div {\n  @media screen {\n  & { color: red; }\n  & { color: red; }\n}\n}",
+    "Two implicit-like rules"
+  );
+  assert_becomes(
+    "div { @media screen { color: red; & { color: red; }",
+    "div {\n  @media screen {\n  color: red;\n  & { color: red; }\n}\n}",
+    "Implicit like rule after decls"
+  );
+  assert_becomes(
+    "div { @media screen { color: red; & { color: blue; }",
+    "div {\n  @media screen {\n  color: red;\n  & { color: blue; }\n}\n}",
+    "Implicit like rule after decls, missing closing braces"
+  );
+  assert_becomes(
+    "div { @media screen { &, p > & { color: blue; }",
+    "div {\n  @media screen {\n  &, p > & { color: blue; }\n}\n}",
+    "Implicit like rule with other selectors"
+  );
+
+  assert_becomes(
+    "div { & { color: red; } }",
+    "div {\n  & { color: red; }\n}",
+    "Implicit-like rule in style rule"
+  );
+
+  // Empty rules (confusingly?) serialize different between style rules
+  // and conditional group rules.
+  assert_unchanged("@media screen {\n}", "Empty conditional rule");
+  assert_unchanged("div { }", "Empty style rule");
+  assert_unchanged("div {\n  @media screen {\n}\n}", "Empty conditional inside style rule");
+  assert_unchanged("@media screen {\n  div { }\n}", "Empty style inside conditional");
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/top-level-is-scope.html b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/top-level-is-scope.html
new file mode 100644
index 000000000000..ab0a806aab4d
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/top-level-is-scope.html
@@ -0,0 +1,30 @@
+<!doctype html>
+<title>Top-level & is treated like :scope</title>
+<link rel="author" title="Steinar H. Gunderson" href="mailto:sesse@chromium.org">
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/">
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+
+<div id="p">
+  <div class="match" id="level1">
+    <div class="match" id="level2"></div>
+  </div>
+</div>
+
+<script>
+  test(() => {
+    let matched = [];
+    for (const elem of p.querySelectorAll('& .match')) {
+      matched.push(elem.getAttribute('id'));
+    }
+    assert_array_equals(matched, ['level1', 'level2']);
+  }, '& as direct ancestor');
+
+  test(() => {
+    let matched = [];
+    for (const elem of p.querySelectorAll('& > .match')) {
+      matched.push(elem.getAttribute('id'));
+    }
+    assert_array_equals(matched, ['level1']);
+  }, '& matches scoped element only, not everything');
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/top-level-parent-pseudo-specificity.html b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/top-level-parent-pseudo-specificity.html
new file mode 100644
index 000000000000..2aa6f2ac663f
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-nesting/top-level-parent-pseudo-specificity.html
@@ -0,0 +1,19 @@
+<!DOCTYPE html>
+<title>CSS Nesting: Specificity of top-level '&'</title>
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1">
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/10196">
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+<style>
+  /* Note: at the top level, '&' matches like ':root'. */
+
+  /* Should have zero specificity: */
+  & { color: red; }
+  /* Should also have zero specificity: */
+  :where(&) { color: green; }
+</style>
+<script>
+  test(() => {
+    assert_equals(getComputedStyle(document.documentElement).color, 'rgb(0, 128, 0)');
+  });
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/support/delete-other-rule-crash.css b/Tests/LibWeb/Text/input/wpt-import/css/support/delete-other-rule-crash.css
new file mode 100644
index 000000000000..13f5d6acf006
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/support/delete-other-rule-crash.css
@@ -0,0 +1,2 @@
+.a {}
+.b { .c {} }