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chromium/third_party/blink/web_tests/external/wpt/css/cssom-view/resources/iframe1.html 

<!DOCTYPE HTML>
<style>
html, body {
    margin: 0;
    padding: 0;
}
#div {
    width: 100px;
    height: 100px;
    background: red;
}
</style>
<div id='div'></div>
<script>
window.onload = window.parent.onFrameLoaded();
</script>