LayoutTests/userscripts/insert-stylesheets-with-pseudo.html

<!DOCTYPE HTML>
<html>
<head>
<script src="../resources/js-test.js"></script>
</head>
<body>
<p>This test requires testRunner and window.internals.</p>
<input id="test" type="range">
<script>
function test()
{
    if (!window.internals)
      return;

    window.testRunner?.dumpAsText();

    let shadow = internals.shadowRoot(document.getElementById("test"));

    window.testElement = shadow.querySelector("[useragentpart='-webkit-slider-runnable-track']");
    shouldNotBe("getComputedStyle(testElement).backgroundColor", "'rgb(255, 0, 0)'");

    debug("");

    debug("Adding author CSS...");
    internals.insertAuthorCSS("input[type=\"range\"]::-webkit-slider-runnable-track { background-color: red; }");
    shouldBe("getComputedStyle(testElement).backgroundColor", "'rgb(255, 0, 0)'");

    debug("");

    debug("Adding user CSS...");
    internals.insertUserCSS("input[type=\"range\"]::-webkit-slider-runnable-track { background-color: blue !important; }");
    shouldBe("getComputedStyle(testElement).backgroundColor", "'rgb(0, 0, 255)'");
}

test();
</script>
</body>
</html>