chromium/third_party/blink/web_tests/fast/dom/HTMLImageElement/resources/srcset_iframe.html

<img id="srcset" srcset="blue_rect.jpg 90w, image-set-4x.png 120w">
<script>
var img = document.getElementById('srcset');
img.onload = function() {
    window.parent.postMessage("loaded", "*");
};
</script>