Gate the Chromium WPT parity legs in CI on Playwright's pinned build

.github/justfile

@@ -34,4 +34,5 @@ rust-checks:
 conformance-checks:
     @just gha::_step conformance-ct::all
     @just gha::_step wpt::parity
+    @just gha::_step wpt::parity-chromium
     @just gha::_step demo::test

.github/workflows/ci.yml

@@ -85,6 +85,29 @@ jobs:
         with:
           path: ${{ steps.pnpm-store.outputs.path }}
           key: pnpm-store-v2-${{ runner.os }}-${{ hashFiles('conformance/driver-ct/jco/pnpm-lock.yaml', 'examples/jco-demo/pnpm-lock.yaml', 'js/componentize/wpt/parity/pnpm-lock.yaml') }}
+      # The WPT parity Chromium leg always runs Playwright's own build,
+      # pinned by playwright-core's version in the parity lockfile, so
+      # losses-chromium.js measures one engine everywhere. The cache key
+      # follows that lockfile; restore-keys lets a playwright bump start
+      # from the previous browsers and download only what changed.
+      - name: Cache Playwright browsers
+        id: playwright-cache
+        uses: actions/cache@v4
+        with:
+          path: ~/.cache/ms-playwright
+          key: playwright-${{ runner.os }}-${{ hashFiles('js/componentize/wpt/parity/pnpm-lock.yaml') }}
+          restore-keys: playwright-${{ runner.os }}-
+      - name: Install Playwright Chromium
+        # On a cache hit the install only verifies the browser is present;
+        # --with-deps (an apt run) is needed only when the browser was
+        # actually (re)downloaded.
+        run: |
+          cd js/componentize/wpt/parity
+          if [ "${{ steps.playwright-cache.outputs.cache-hit }}" = "true" ]; then
+            npx playwright-core install chromium
+          else
+            npx playwright-core install --with-deps chromium
+          fi
       - run: just gha::conformance-checks
         # The jco-browser adapter finds the runner's preinstalled Chrome via
         # its candidate paths; CHROME_PATH overrides if ever needed.

js/componentize/wpt/README.md

@@ -31,9 +31,12 @@ unbundled. Each engine ratchets separately (`parity/losses-chromium.js`,
 re-recorded with `just wpt::update-losses-chromium`): a loss set is a fact
 about one engine's baseline.
 
-The Chromium gate is not yet in CI: it needs a Chrome 137+ binary on the
-runner. Test names are engine-independent by construction — `wpt-env.js`
-shadows `location` with a fixed stub in every leg, so names never embed a
+The Chromium legs always run Playwright's own build, pinned by
+playwright-core's version in the parity lockfile, so the loss set
+measures one engine everywhere — local runs and CI alike; a Chromium
+behavior shift arrives only with a deliberate playwright bump. Test
+names are engine-independent by construction — `wpt-env.js` shadows
+`location` with a fixed stub in every leg, so names never embed a
 per-run origin.
 
 ## Vendoring policy

js/componentize/wpt/justfile

@@ -30,8 +30,10 @@ update-losses: _artifacts
 # baseline measures Chromium's own WebSocket, the round trip runs the
 # browser-profile transpile against websocket-jco in the environment it
 # targets. Ratchets separately in parity/losses-chromium.js — a loss set
-# is a fact about one engine's baseline. Not yet in CI: needs a Chrome
-# 137+ binary on the runner.
+# is a fact about one engine's baseline. The browser is always
+# Playwright's pinned Chromium build (install once with
+# `npx playwright-core install --with-deps chromium` from parity/), so
+# the loss set measures one engine everywhere.
 parity-chromium: _artifacts-web
     {{timeout_cmd}} node {{root}}/js/componentize/wpt/parity/run-browser.mjs
 

js/componentize/wpt/parity/run-browser.mjs

@@ -14,7 +14,11 @@
 // echo server directly: WebSocket is not subject to CORS, and a localhost
 // `http:` page may open `ws:` connections.
 //
-// jco's async ABI needs JSPI; Chrome ships it enabled from 137 onward.
+// The browser is always Playwright's own Chromium build, pinned by
+// playwright-core's version (the parity lockfile), so losses-chromium.js
+// measures one engine everywhere — local runs and CI alike. Install it
+// once with `npx playwright-core install --with-deps chromium` (from this
+// directory). jco's async ABI needs JSPI; Chromium ships it.
 //
 // Usage: node run-browser.mjs [--update]
 
@@ -26,7 +30,6 @@ import { fileURLToPath } from "node:url";
 
 import { chromium } from "playwright-core";
 
-import { findChrome } from "../../../../scripts/chrome.mjs";
 import { spawnEchod } from "../../../../conformance/server/echod.mjs";
 
 const HERE = dirname(fileURLToPath(import.meta.url));
@@ -99,18 +102,12 @@ async function main() {
     throw new Error(`missing browser-profile transpile in ${join(HERE, "generated-web")}; run "npm run transpile:web" first`);
   }
 
-  const executablePath = await findChrome();
-  if (!executablePath) {
-    throw new Error("no Chrome/Chromium binary found; set CHROME_PATH to a Chrome 137+ executable");
-  }
-
   const echod = await spawnEchod(join(REPO_ROOT, "target", "debug", "conformance-echod"));
   const server = await startServer();
   const base = `http://127.0.0.1:${server.address().port}`;
   process.stderr.write(`echo server at ${echod.base}; page served from ${base}\n`);
 
   const browser = await chromium.launch({
-    executablePath,
     headless: true,
     args: ["--no-sandbox", "--disable-dev-shm-usage"],
   });

scripts/chrome.mjs

@@ -1,6 +1,7 @@
-// Chrome/Chromium discovery shared by every headless-browser runner (the
-// conformance jco-browser leg and the WPT parity browser legs).
-// Node-only.
+// Chrome/Chromium discovery for the conformance jco-browser leg, which
+// prefers a system Chrome. (The WPT parity browser legs do not use this:
+// they always launch Playwright's own pinned build, so the recorded loss
+// set measures one engine everywhere.) Node-only.
 import { access, readdir } from "node:fs/promises";
 import { join } from "node:path";