HTML: timing of iframe's load event

For whatwg/html#4965.

html/semantics/embedded-content/the-iframe-element/load-event-timing.window.js

@@ -0,0 +1,58 @@
+async_test(t => {
+  const frame = document.createElement("iframe");
+  t.add_cleanup(() => frame.remove());
+  let happened = []
+  frame.onload = t.step_func(() => happened.push("load"));
+  document.body.append(frame);
+  happened.push("append");
+  // Use timeout for assert in case there's multiple load events erroneously
+  t.step_timeout(() => {
+    assert_array_equals(happened, ["load", "append"]);
+    t.done();
+  }, 500);
+}, "<iframe> without src and load event");
+
+async_test(t => {
+  const frame = document.createElement("iframe");
+  t.add_cleanup(() => frame.remove());
+  let happened = []
+  frame.onload = t.step_func(() => happened.push("load"));
+  frame.src = "about:blank";
+  document.body.append(frame);
+  happened.push("append");
+  // Use timeout for assert in case there's multiple load events erroneously
+  t.step_timeout(() => {
+    assert_array_equals(happened, ["load", "append"]);
+    t.done();
+  }, 500);
+}, "<iframe> with about:blank src and load event");
+
+async_test(t => {
+  const frame = document.createElement("iframe");
+  t.add_cleanup(() => frame.remove());
+  let happened = false;
+  frame.onload = t.step_func_done(() => assert_true(happened));
+  frame.src = URL.createObjectURL(new Blob([""], { type: "text/html" }));
+  document.body.append(frame);
+  happened = true;
+}, "<iframe> with blob: URL src and load event");
+
+async_test(t => {
+  const frame = document.createElement("iframe");
+  t.add_cleanup(() => frame.remove());
+  let happened = false;
+  frame.onload = t.step_func_done(() => assert_true(happened));
+  frame.src = "data:text/html,";
+  document.body.append(frame);
+  happened = true;
+}, "<iframe> with data: URL src and load event");
+
+async_test(t => {
+  const frame = document.createElement("iframe");
+  t.add_cleanup(() => frame.remove());
+  let happened = false;
+  frame.onload = t.step_func_done(() => assert_true(happened));
+  frame.srcdoc = "";
+  document.body.append(frame);
+  happened = true;
+}, "<iframe> with srcdoc and load event");