chromium/third_party/blink/web_tests/external/wpt/infrastructure/reftest-wait.html

<!DOCTYPE html>
<html class="reftest-wait">
<title>Test infrastructure should support 'reftest-wait'</title>
<link rel="match" href="reftest-wait-ref.html">
<style>
.marker {
    margin-bottom: 10px;
    background: red;
    height: 50px;
    width: 50px;
}
</style>
<script>
setTimeout(function() {
    document.querySelector(".marker").style.background = 'green';
    document.documentElement.classList.remove("reftest-wait");
}, 1000);
</script>
<div class="marker"></div>
<div>The box above should be green.</div>
</html>