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chromium/third_party/blink/web_tests/fast/forms/label/detached-document.html 

<!DOCTYPE html>
<label id="label">
<script>
var label = document.getElementById("label");
var doc = document.implementation.createDocument("", null);
doc.adoptNode(label);
var mouseEvent = document.createEvent('MouseEvent');
mouseEvent.initMouseEvent("click", 56644);
label.dispatchEvent(mouseEvent)
</script>