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chromium/third_party/blink/web_tests/external/wpt/navigation-api/navigation-history-entry/resources/post-entries-length-to-top.html 

<!doctype html>
<script src="/resources/testharness.js"></script>
<script>
// Wait for after the load event so that the navigation doesn't get converted
// into a replace navigation.
window.onload = () => step_timeout(() => top.postMessage(navigation.entries().length, "*"), 0);
</script>