HTML: add test for document.open() and URL fragment

For whatwg/html#3970.

Co-authored-by: Timothy Gu <[email protected]>

html/webappapis/dynamic-markup-insertion/opening-the-input-stream/resources/url-frame.html

@@ -0,0 +1,9 @@
+<script>
+onload = () => {
+  const beforeURL = document.URL;
+  document.open();
+  const afterURL = document.URL;
+  document.close();
+  parent.testDone(beforeURL, afterURL);
+}
+</script>

html/webappapis/dynamic-markup-insertion/opening-the-input-stream/url-fragment.window.js

@@ -0,0 +1,24 @@
+async_test(t => {
+  const frame = document.body.appendChild(document.createElement("iframe")),
+        urlSansHash = document.URL;
+  assert_equals(frame.contentDocument.URL, "about:blank");
+  assert_equals(frame.contentWindow.location.href, "about:blank");
+  self.onhashchange = t.step_func_done(() => {
+    frame.contentDocument.open();
+    assert_equals(frame.contentDocument.URL, urlSansHash);
+    assert_equals(frame.contentWindow.location.href, urlSansHash);
+  });
+  self.location.hash = "heya";
+  t.step_timeout(t.unreached_func("timeout"), 2000);
+}, "document.open() and document's URL containing a fragment");
+
+window.testDone = undefined;
+async_test(t => {
+  const frame = document.body.appendChild(document.createElement("iframe"))
+  frame.src = "resources/url-frame.html#heya";
+  window.testDone = t.step_func_done((beforeURL, afterURL) => {
+    assert_equals(beforeURL, frame.src);
+    assert_equals(afterURL, frame.src);
+    assert_equals(frame.contentDocument.URL, frame.src);
+  });
+}, "document.open() and document's URL containing a fragment (entry is current)");