<html>
<head>
<style>
:root { --bgcolor: green; background: var(--bgcolor); }
</style>
<body>
<script>
/* https://www.w3.org/TR/css-variables/#variables-in-shorthands
   "Pending-substitution values must be serialized as the empty string, if an API allows them to be observed."
   This means getting the shorthand value for background directly from the style rule or element.style
   should produce an empty string if the property contains a variable. */
document.write(document.styleSheets[0].cssRules[0].style.getPropertyValue("background"))
</script>
</body>
</html.