css/css-variables/variable-reference-variable.html

<!DOCTYPE html>
<html>
<head>
    <title>Parse, store, and serialize CSS variable referencing another CSS variable</title>

    <meta rel="author" title="Kevin Babbitt">
    <meta rel="author" title="Greg Whitworth">
    <link rel="author" title="Microsoft Corporation" href="http://microsoft.com" />
    <link rel="help" href="http://www.w3.org/TR/css-variables-1/#serializing-custom-props">

    <script src="/resources/testharness.js"></script>
    <script src="/resources/testharnessreport.js"></script>
</head>
<body>
    <div id="test1" style="--prop1: var(--prop2);"></div>
    <div id="test2" style="--prop1: var(--prop2, var(--prop3));"></div>

    <script type="text/javascript">
        "use strict";

        var testcases = [
            { elementId: "test1",   expectedPropertyValue: "var(--prop2)" },
            { elementId: "test2",   expectedPropertyValue: "var(--prop2, var(--prop3))" },
        ];

        testcases.forEach(function (testcase) {
            test( function () {
                var testElement = document.getElementById(testcase.elementId);
                var actualPropertyValue = testElement.style.getPropertyValue("--prop1").trim();
                assert_equals(actualPropertyValue, testcase.expectedPropertyValue);
            }, testcase.cssText);
        });
    </script>

</body>
</html>