<head>
<style>
@import url("data:text/css,#test{}");
</style>
<script>
if (window.testRunner)
    testRunner.dumpAsText();

function test() {
    if (document.styleSheets[0].cssRules[0].styleSheet.parentStyleSheet == document.styleSheets[0])
        document.getElementById('test').innerHTML = 'PASS';
}
</script>
<body onload='test()'>
Test that StyleSheet.parentStyleSheet works properly. You should see "PASS" below.
<div id="test">FAIL</div>