css/css-variables/variable-transitions-value-before-transition-property-variable.html

<!DOCTYPE html>
<html>
<head>
    <title>CSS Variables - Transitions - Variable value specified before transition; variable specified by name</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/#syntax">

    <script src="/resources/testharness.js"></script>
    <script src="/resources/testharnessreport.js"></script>
    <style>
        #target
        {
            --value: blue;
            color: var(--value);
        }
        #target
        {
            transition-property: --value;
            transition-duration: 1s;
        }
        #target.changed
        {
            --value: green;
        }
    </style>
</head>
<body>

    <div id="target">This text should transition from blue to green over a period of 1 second.</div>

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

        // Force an initial layout pass
        document.documentElement.offsetHeight;

        test(function() {
            assert_equals(window.getComputedStyle(document.getElementById("target")).getPropertyValue("--value").trim(), "blue", "--value is blue before transition runs");
            }, "Verify CSS variable value before transition");

        test(function() {
            assert_equals(window.getComputedStyle(document.getElementById("target")).getPropertyValue("color").trim(), "rgb(0, 0, 255)", "color is blue before transition runs");
            }, "Verify substituted color value before transition");

        var afterAnimationVariableTest = async_test("Verify CSS variable value after transition");
        document.getElementById("target").addEventListener("transitionend", afterAnimationVariableTest.step_func_done(function() {
            assert_equals(window.getComputedStyle(document.getElementById("target")).getPropertyValue("--value").trim(), "green", "--value is green after transition runs")
            }));

        var afterAnimationColorTest = async_test("Verify substituted color value after transition");
        document.getElementById("target").addEventListener("transitionend", afterAnimationColorTest.step_func_done(function() {
            assert_equals(window.getComputedStyle(document.getElementById("target")).getPropertyValue("color").trim(), "rgb(0, 128, 0)", "color is green after transition runs")
            }));

        // Trigger transition
        document.getElementById("target").className = "changed";

        // Force another layout pass
        document.documentElement.offsetHeight;
    </script>

</body>
</html>