gger the
  // mutation observer don't crash.
  // https://github.com/lightpanda-io/browser/issues/550
  var node2 = $("#p2");
  new MutationObserver(() => {
      node2.innerText = 'a';
  }).observe($('#d2'), { subtree:true, childList:true });
  node2.innerText = "2";
  testing.eventually(() => testing.expectEqual('a', node2.innerText));

  var node3 = $("#p3");
  var attrWatch = 0;
  new MutationObserver(() => {
      attrWatch++;
  }).observe($('#d3'), { attributeFilter: ["name"], subtree: true });
  node