HTML: <img>'s historical progress events

For whatwg/html#4842.

Co-Authored-By: Philip Jägenstedt <philip@foolip.org>

html/semantics/embedded-content/the-img-element/historical-progress-event.window.js

@@ -0,0 +1,16 @@
+async_test(t => {
+  const img = new Image();
+  t.add_cleanup(() => img.remove());
+  img.onloadstart = img.onprogress = img.onloadend = t.unreached_func("progress event fired");
+  img.onload = t.step_func_done(e => {
+    assert_true(e instanceof Event);
+    assert_false(e instanceof ProgressEvent);
+  });
+  img.src = "/images/rrgg-256x256.png";
+  document.body.append(img);
+}, "<img> does not support ProgressEvent or loadstart/progress/loadend");
+
+test(t => {
+  assert_equals(document.body.onloadend, undefined);
+  assert_equals(window.onloadend, undefined);
+}, "onloadend is not exposed");