LayoutTests/userscripts/insert-stylesheets.html

<!DOCTYPE HTML>
<html>
<head>
<style>
  .styled {
      background-color: red;
      width: 0;
      height: 100px;
  }
</style>
<script src="../resources/js-test-pre.js"></script>
</head>
<body>
<p>This test requires testRunner and window.internals.</p>
<div id="test" class="styled"></div>
<script>
function test()
{
    if (!window.testRunner)
        return;
    testRunner.dumpAsText();
    window.testElement = document.getElementById("test");
    shouldBe("getComputedStyle(testElement).backgroundColor", "'rgb(255, 0, 0)'");
    shouldBe("testElement.offsetWidth", "0");

    if (!window.internals)
        return;

    // The author style above should override this user style.
    internals.insertUserCSS("body .styled { background-color: green; width: 100px; }");
    shouldBe("getComputedStyle(testElement).backgroundColor", "'rgb(255, 0, 0)'");
    shouldBe("testElement.offsetWidth", "0");

    // Since this style is more specific, it should override the original author style above.
    internals.insertAuthorCSS("body .styled { background-color: green; width: 100px; }");
    shouldBe("getComputedStyle(testElement).backgroundColor", "'rgb(0, 128, 0)'");
    shouldBe("testElement.offsetWidth", "100");
}

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