@@ -0,0 +1,35 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const fixtures = require('../common/fixtures');
+const { spawnSync } = require('child_process');
+
+if (process.env.NODE_TEST_WPT_MODIFIER_PROBE === '1') {
+  const { WPTRunner } = require('../common/wpt');
+  const runner = new WPTRunner('html/webappapis/atob');
+  runner.setScriptModifier(common.mustCall((script) => {
+    assert.strictEqual(script.filename,
+                       fixtures.path('wpt', 'html/webappapis/atob', 'base64.any.js'));
+    script.filename += '.modified';
+    script.code = `test(() => {
+      assert_true(new Error().stack.includes(${JSON.stringify(`${script.filename}:`)}));
+    }, 'modified script retains its filename');`;
+  }));
+  runner.runJsTests();
+} else {
+  // eslint-disable-next-line no-unused-vars
+  const { WPT_REPORT, ...env } = { ...process.env, NODE_TEST_WPT_MODIFIER_PROBE: '1' };
+  const result = spawnSync(process.execPath, [__filename, 'base64.any.js'], {
+    env,
+    encoding: 'utf8',
+    timeout: common.platformTimeout(10_000),
+  });
+  const { error, status, stdout, stderr } = result;
+  assert.ifError(error);
+  assert.strictEqual(status, 0, stdout + stderr);
+  const results = stdout.split('\n').filter((line) => line.startsWith('[PASS]'));
+  assert.deepStrictEqual(results, [
+    '[PASS] modified script retains its filename',
+  ]);
+}
@@ -1,9 +1,31 @@
 'use strict';
 
+const assert = require('assert');
+const { basename } = require('path');
 const { WPTRunner } = require('../common/wpt');
 
 const runner = new WPTRunner('user-timing');
 
 runner.pretendGlobalThisAs('Window');
 
+runner.setScriptModifier((script) => {
+  if (basename(script.filename) !== 'mark.any.js') return;
+
+  // A scheduling pause between mark() and now() can exceed any fixed
+  // tolerance. Check that the mark falls between the surrounding clock
+  // readings instead, leaving the upstream fixture unchanged.
+  // https://github.com/nodejs/node/issues/40449
+  function replace(from, to) {
+    assert(script.code.includes(from), `Unexpected contents of ${script.filename}`);
+    script.code = script.code.replaceAll(from, to);
+  }
+
+  replace('var expectedTimes = new Array();',
+          'var beforeTimes = [];\nvar expectedTimes = new Array();');
+  replace('self.performance.mark("mark");',
+          'beforeTimes.push(self.performance.now());\n    self.performance.mark("mark");');
+  replace('assert_approx_equals(entries[index].startTime, expectedTimes[index], testThreshold);',
+          'assert_between_inclusive(entries[index].startTime, beforeTimes[index], expectedTimes[index]);');
+});
+
 runner.runJsTests();
@@ -5,8 +5,6 @@ prefix wpt
 # sample-test                        : PASS,FLAKY
 
 [true] # This section applies to all platforms
-# https://github.com/nodejs/node/issues/40449
-test-user-timing: PASS,FLAKY
 
 [$system==win32]