LayoutTests/fast/css/custom-properties/rule-property-priority.html - WebKit - Git at Google

 <html>
 <head>
 <style>
 :root { --one: 10px !important; --one:50px; }
 </style>
 <body>
 <script>
 document.write(document.styleSheets[0].cssRules[0].style.getPropertyValue("--one") + " " + document.styleSheets[0].cssRules[0].style.getPropertyPriority("--one"))
 </script>
 </body>
 </html.
	<html>
	<head>
	<style>
	:root { --one: 10px !important; --one:50px; }
	</style>
	<body>
	<script>
	document.write(document.styleSheets[0].cssRules[0].style.getPropertyValue("--one") + " " + document.styleSheets[0].cssRules[0].style.getPropertyPriority("--one"))
	</script>
	</body>
	</html.