css/cssom/MutationObserver-style.html

<!doctype html>
<meta charset=utf-8>
<title>Observer notifications when updating styles</title>
<link rel="help" href="https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-setproperty">
<script src=/resources/testharness.js></script>
<script src=/resources/testharnessreport.js></script>
<body>
<script>
  "use strict";

  promise_test(async () => {
    let called = 0;
    const el = document.createElement("div");
    document.body.appendChild(el);
    const m = new MutationObserver(() => {
      called++;
    });
    m.observe(el, { attributes: true });
    el.style.height = "100px";
    await Promise.resolve();
    assert_equals(called, 1, "times callback called");
    el.style.height = "100px";
    await Promise.resolve();
    assert_equals(called, 1, "times callback called");
  }, "Updating style property with the same value does not trigger an observation callback");

  promise_test(async () => {
    let called = 0;
    const el = document.createElement("div");
    document.body.appendChild(el);
    const m = new MutationObserver(() => {
      called++;
    });
    m.observe(el, { attributes: true });
    el.style.cssText = "height:100px";
    await Promise.resolve();
    assert_equals(called, 1, "times callback called");
    el.style.cssText = "height:100px";
    await Promise.resolve();
    assert_equals(called, 2, "times callback called");
  }, "Updating cssText triggers an observation callback even if the value is the same");
</script>