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chromium/third_party/blink/web_tests/fast/forms/fieldset/fieldset-disable.html 

<!DOCTYPE html>
<html>
<body>
<form>
	<fieldset id="a" disabled="true">
		<input id="b" type="file" disabled="true">
		<br />
		<input id="c" type="submit" disabled="true">
	</fieldset>
</form>
</body>
<script>
	document.getElementById("b").removeAttribute("disabled");
	document.getElementById("c").removeAttribute("disabled");
	document.getElementById("a").removeAttribute("disabled");
</script>
</html>