This test passes if it doesn't crash.
<script>
if (window.testRunner)
    testRunner.dumpAsText();

const style = document.createElement("style");
document.head.append(style);

style.sheet.insertRule('@import url(data:,foo)');
style.sheet.cssRules[0];
style.sheet.deleteRule(0);
style.sheet.insertRule('@import url(data:,bar)');
</script>
