third_party/WebKit/LayoutTests/fast/dom/custom/callback-timing.html

<script src='../../../resources/js-test.js'></script>
<x-foo></x-foo>
<script>
description('Ensure that custom element callback fires before DOMContentLoaded event');

var eventlog = [];

addEventListener('DOMContentLoaded', function(e) {
    eventlog.push('DOMContentLoaded event');
});
addEventListener('load', function(e) {
    eventlog.push('load event');

    shouldBe("eventlog[0]", "'created x-foo'");
    shouldBe("eventlog[1]", "'DOMContentLoaded event'");
    shouldBe("eventlog[2]", "'load event'");
});
var proto = Object.create(HTMLElement.prototype);
proto.createdCallback = function() {
    eventlog.push('created ' + this.localName);
};
document.registerElement('x-foo', {prototype: proto});
</script>