yId("thebody");
    assert_equals(thebody.id, "thebody");
    thebody.style.overflowY="scroll";
    assert_equals(document.body.style.overflowY, "scroll", "Could not set document.body.style.overflowY to 'scroll'.");
    // Body and document now both have overflow != visible
    // => body `potentially scrollable`

    // In quirks, when body is not `potentially scrollable`
    //    document.scrollingElment returns the body, otherwise null
    // https://drafts.csswg.org/cssom-view/#dom-document-scrollingel