div style="writing-mode: sideways-lr"
  data-expected="sideways-lr"></div>

<script>
Array.prototype.forEach.call(document.querySelectorAll("[data-expected]"), function (element) {
  test(function () {
    var actual = getComputedStyle(element).writingMode;
    assert_equals(actual, element.dataset.expected);
  }, element.title || element.getAttribute("style"));
});
</script>
