LayoutTests/userscripts/user-stylesheet-invalidate.html

<!DOCTYPE HTML>
<html>
<head>
<style>
* { color: red; }
</style>
<script>
if (window.testRunner) {
    testRunner.dumpAsText();
    testRunner.addUserStyleSheet("div { width:0; }", true);
}
</script>
</head>
<body>
This test requires layout test runner.
<div id="target">FAIL</div>
<script>
var style = document.createElement("style");
style.innerText = "* { color: blue; }";
document.head.insertBefore(style, document.getElementsByTagName("style")[0]);
if (document.getElementById('target').offsetWidth == 0)
    document.getElementById('target').innerHTML = 'PASS';
</script>
</body>
</html>