LibWeb/CSS: Stop assuming CSSNestedDeclarations's parent is CSSStyleRule

This will no longer be true once we implement at-rules nested inside
style rules, for example:

```css
.foo {
  color: yellow;

  @media (min-width: 800px) {
    color: orange;
  }
}
```

Userland/Libraries/LibWeb/CSS/CSSNestedDeclarations.cpp

@@ -7,6 +7,7 @@
 #include "CSSNestedDeclarations.h"
 #include <LibWeb/Bindings/CSSNestedDeclarationsPrototype.h>
 #include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/CSS/CSSStyleRule.h>
 
 namespace Web::CSS {
 
@@ -34,13 +35,30 @@ void CSSNestedDeclarations::visit_edges(Cell::Visitor& visitor)
 {
     Base::visit_edges(visitor);
     visitor.visit(m_declaration);
+    visitor.visit(m_parent_style_rule);
 }
 
 CSSStyleDeclaration* CSSNestedDeclarations::style()
 {
     return m_declaration;
 }
 
+CSSStyleRule const& CSSNestedDeclarations::parent_style_rule() const
+{
+    if (m_parent_style_rule)
+        return *m_parent_style_rule;
+
+    for (auto* parent = parent_rule(); parent; parent = parent->parent_rule()) {
+        if (is<CSSStyleRule>(parent)) {
+            m_parent_style_rule = static_cast<CSSStyleRule const*>(parent);
+            return *m_parent_style_rule;
+        }
+    }
+
+    dbgln("CSSNestedDeclarations has no parent style rule!");
+    VERIFY_NOT_REACHED();
+}
+
 String CSSNestedDeclarations::serialized() const
 {
     // NOTE: There's no proper spec for this yet, only this note:
@@ -50,4 +68,10 @@ String CSSNestedDeclarations::serialized() const
     return m_declaration->serialized();
 }
 
+void CSSNestedDeclarations::clear_caches()
+{
+    Base::clear_caches();
+    m_parent_style_rule = nullptr;
+}
+
 }

Userland/Libraries/LibWeb/CSS/CSSNestedDeclarations.h

@@ -23,14 +23,18 @@ class CSSNestedDeclarations final : public CSSRule {
 
     CSSStyleDeclaration* style();
 
+    CSSStyleRule const& parent_style_rule() const;
+
 private:
     CSSNestedDeclarations(JS::Realm&, PropertyOwningCSSStyleDeclaration&);
 
     virtual void initialize(JS::Realm&) override;
     virtual void visit_edges(Cell::Visitor&) override;
     virtual String serialized() const override;
+    virtual void clear_caches() override;
 
     JS::NonnullGCPtr<PropertyOwningCSSStyleDeclaration> m_declaration;
+    JS::GCPtr<CSSStyleRule const> mutable m_parent_style_rule;
 };
 
 template<>

Userland/Libraries/LibWeb/CSS/StyleComputer.cpp

@@ -111,7 +111,7 @@ SelectorList const& MatchingRule::absolutized_selectors() const
     if (rule->type() == CSSRule::Type::Style)
         return static_cast<CSSStyleRule const&>(*rule).absolutized_selectors();
     if (rule->type() == CSSRule::Type::NestedDeclarations)
-        return static_cast<CSSStyleRule const&>(*rule->parent_rule()).absolutized_selectors();
+        return static_cast<CSSNestedDeclarations const&>(*rule).parent_style_rule().absolutized_selectors();
     VERIFY_NOT_REACHED();
 }
 
@@ -120,7 +120,7 @@ FlyString const& MatchingRule::qualified_layer_name() const
     if (rule->type() == CSSRule::Type::Style)
         return static_cast<CSSStyleRule const&>(*rule).qualified_layer_name();
     if (rule->type() == CSSRule::Type::NestedDeclarations)
-        return static_cast<CSSStyleRule const&>(*rule->parent_rule()).qualified_layer_name();
+        return static_cast<CSSNestedDeclarations const&>(*rule).parent_style_rule().qualified_layer_name();
     VERIFY_NOT_REACHED();
 }
 
@@ -2462,7 +2462,7 @@ NonnullOwnPtr<StyleComputer::RuleCache> StyleComputer::make_rule_cache_for_casca
                 if (rule.type() == CSSRule::Type::Style)
                     return static_cast<CSSStyleRule const&>(rule).absolutized_selectors();
                 if (rule.type() == CSSRule::Type::NestedDeclarations)
-                    return static_cast<CSSStyleRule const&>(*rule.parent_rule()).absolutized_selectors();
+                    return static_cast<CSSNestedDeclarations const&>(rule).parent_style_rule().absolutized_selectors();
                 VERIFY_NOT_REACHED();
             }();
             for (CSS::Selector const& selector : absolutized_selectors) {