LayoutTests/fast/css/important-js-override.html

<div id="a" style="background-color: green !important">The background of this element should be green. It is </div>
<script>
    if (window.testRunner)
        testRunner.dumpAsText();
    var a = document.getElementById("a");
    a.style.backgroundColor = "red";
    a.innerHTML += a.style.backgroundColor;
</script>