diff --git a/README.md b/README.md
index df4c4f58fa..311c3ac381 100644
--- a/README.md
+++ b/README.md
@@ -9,8 +9,7 @@
 Requires Node.js (`node`), Python (`python3`), and Java (`java`). It is known to work in these environments but should work in others:
 
  - Windows 10, Python 3.11.7 (Microsoft Store), Node.js 20.10.0 (nodejs.org installer), Java 11 (Temurin-11.0.21+9)
-
-Python 3.12 currently does not work.
+ - Windows 10, Python 3.12.1 (Microsoft Store), Node.js 20.10.0 (nodejs.org installer), Java 11 (Temurin-11.0.21+9)
 
 Install dependencies:
 
@@ -18,9 +17,9 @@ Install dependencies:
 npm ci
 ```
 
-The playground to use for local testing is tests/vertical_playground.html.
+Open tests/vertical_playground.html in a browser for local testing. You don't need to rebuild compressed versions for most changes.
 
-To build, run:
+To re-build compressed versions, run:
 
 ```sh
 npm run prepublish
diff --git a/build.py b/build.py
index 0afb50901b..67bc097db3 100755
--- a/build.py
+++ b/build.py
@@ -636,13 +636,23 @@ def exclude_horizontal(item):
   # Run all tasks in parallel threads.
   # Uncompressed is limited by processor speed.
   # Compressed is limited by network and server speed.
-  # Vertical:
-  Gen_uncompressed(search_paths_vertical, True, closure_env).start()
-  # Horizontal:
-  Gen_uncompressed(search_paths_horizontal, False, closure_env).start()
-
-  # Compressed forms of vertical and horizontal.
-  Gen_compressed(search_paths_vertical, search_paths_horizontal, closure_env).start()
-
-  # This is run locally in a separate thread.
-  # Gen_langfiles().start()
+  threads = [
+    # Vertical:
+    Gen_uncompressed(search_paths_vertical, True, closure_env),
+    # Horizontal:
+    Gen_uncompressed(search_paths_horizontal, False, closure_env),
+    # Compressed forms of vertical and horizontal.
+    Gen_compressed(search_paths_vertical, search_paths_horizontal, closure_env),
+
+    # This is run locally in a separate thread.
+    # Gen_langfiles()
+  ]
+
+  for thread in threads:
+    thread.start()
+
+  # Need to wait for all threads to finish before the main process ends as in Python 3.12,
+  # once the main interpreter is being shutdown, trying to spawn more child threads will
+  # raise "RuntimeError: can't create new thread at interpreter shutdown"
+  for thread in threads:
+    thread.join()