<html>
<head>
<style>
:root { --one: 10px; --two: 20px; }
</style>
<body>
<script>
document.write(document.styleSheets[0].cssRules[0].style.getPropertyCSSValue("--one").cssText + " " + document.styleSheets[0].cssRules[0].style.getPropertyCSSValue("--two").cssText)
</script>
</body>
</html.