LayoutTests/fast/css/simple-color-change.html

<style>
div { color: red; }
.green { color: green; }
</style>
<div id=test>Green</div>
<script>
if (window.testRunner)
    testRunner.waitUntilDone();

setTimeout(() => {
    test.classList.add("green");

    if (window.testRunner)
        testRunner.notifyDone();
}, 20);
</script>