The HTML parser synchronously creates custom elements

third_party/WebKit/LayoutTests/custom-elements/spec/resources/custom-elements-helpers.js

 function create_window_in_test(t, srcdoc) {
   let p = new Promise((resolve) => {
     let f = document.createElement('iframe');
     f.srcdoc = srcdoc ? srcdoc : '';
     f.onload = (event) => {
       let w = f.contentWindow;
-      t.add_cleanup(() => f.remove());
+      t.add_cleanup(() => f.parentNode && f.remove());
       resolve(w);
     };
     document.body.appendChild(f);
   });
   return p;
 }
 
 function test_with_window(f, name, srcdoc) {
   promise_test((t) => {
     return create_window_in_test(t, srcdoc)
@@ skipping 18 matching lines @@
 
 function assert_array_equals_callback_invocations(actual, expected, description) {
   assert_equals(actual.length, expected.length);
   for (let len=actual.length, i=0; i<len; ++i) {
     let callback = expected[i][0];
     assert_equals(actual[i][0], expected[i][0], callback + ' callback should be invoked');
     assert_equals(actual[i][1], expected[i][1], callback + ' should be invoked on the element ' + expected[i][1]);
     assert_array_equals(actual[i][2], expected[i][2], callback + ' should be invoked with the arguments ' + expected[i][2]);
   }
 }