LayoutTests/fast/css/lang-document-element-flag.html

<!DOCTYPE html>
<script src="../../resources/js-test.js"></script>
<body>
<script>
description("Tests that the document element has effectiveLangKnownToMatchDocumentElement flag set to true");

var frame, iframeDocElement, newDocElement, body;

if (window.internals) {
    shouldBeTrue('internals.effectiveLangKnownToMatchDocumentElement(document.documentElement)');

    frame = document.createElement("iframe");
    frame.srcdoc = "<!DOCTYPE html><html lang=en><body>";
    document.body.append(frame);

    frame.onload = function() {
        iframeDocElement = frame.contentDocument.documentElement;
        shouldBeTrue('internals.effectiveLangKnownToMatchDocumentElement(iframeDocElement)');
        iframeDocElement.remove();
        shouldBeFalse('internals.effectiveLangKnownToMatchDocumentElement(iframeDocElement)');
        body = iframeDocElement.querySelector("body");
        shouldBeTrue('internals.effectiveLangKnownToMatchDocumentElement(body)');
        newDocElement = frame.contentDocument.createElement("html");
        newDocElement.lang = "de";
        frame.contentDocument.append(newDocElement);
        shouldBeTrue('internals.effectiveLangKnownToMatchDocumentElement(newDocElement)');
        frame.remove();
        finishJSTest();
    };

    var jsTestIsAsync = true;
} else
    testFailed("This test requires window.internals");
</script>
</body>