diff --git a/.github/workflows/size_report.yml b/.github/workflows/size_report.yml
new file mode 100644
index 0000000000..c542bff3e2
--- /dev/null
+++ b/.github/workflows/size_report.yml
@@ -0,0 +1,21 @@
+# This workflow computes the size of a CDN build's output on all Javascript files.
+# Reported file sizes are after the result of gzip compression.
+# Compression action used: https://github.com/preactjs/compressed-size-action
+
+name: Size Report (gzip)
+
+on: [pull_request]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+      - name: Compute Compressed Size
+        uses: preactjs/compressed-size-action@v2
+        with:
+          build-script: "build-cdn"
+          compression: "gzip"
+          pattern: "./build/{*.min.js,es/*.min.js,languages/*.min.js,es/languages/*.min.js}"
diff --git a/CHANGES.md b/CHANGES.md
index da9423a0c6..59324b44b6 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -15,6 +15,10 @@ Grammars:
   - fix(clojure) Remove inconsistent/broken highlighting for metadata
   - enh(clojure) Add `punctuation` mode for commas.
 
+Developer Tools:
+
+- (chore) add gzip size compression report (#3400) [Bradley Mackey][]
+
 [Richard Gibson]: https://github.com/gibson042
 [Bradley Mackey]: https://github.com/bradleymackey
 [Björn Ebbinghaus]: https://github.com/MrEbbinghaus