chromium/third_party/blink/web_tests/fast/dom/css-insert-import-rule.html

<head>
<style>
@import url("resources/css-insert-import-rule-red.css");
div {
    width: 100px;
    height: 100px;
}
</style>

<script>
document.styleSheets[0].insertRule('@import url("resources/css-insert-import-rule.css");', 0);
</script>

</head>
<body>
<div></div>
</body>