<html>
<script>
if (window.testRunner)
    testRunner.dumpAsText();
</script>
<style>html,tr,img, table,media,body, li, em:nth-child(5){
    height: 500px
}
</style>
<script>
function load() {
    var cssRules = window.getMatchedCSSRules(document.documentElement);
    cssRules[0].selectorText = 'a,td';
}
</script>
This test passes if it doesn't crash.
<iframe onload=load()>
</html>