HTML: mark typemustmatch historical

Apart from Firefox nobody adopted this and it creates a smallish cross-origin leak. whatwg/html#4590 changes the HTML standard.

html/dom/elements-embedded.js

@@ -59,7 +59,6 @@ var embeddedElements = {
     // Conforming
     data: "url",
     type: "string",
-    typeMustMatch: "boolean",
     name: "string",
     useMap: "string",
     width: "string",

html/semantics/embedded-content/the-object-element/historical.html

@@ -12,4 +12,22 @@
     elm();
   });
 }, 'object legacycaller should not be supported');
+
+test(() => {
+  const obj = document.createElement("object");
+  assert_false("typeMustMatch" in obj);
+}, "object's typeMustMatch IDL attribute should not be supported");
+
+async_test(t => {
+  const obj = document.createElement("object");
+  t.add_cleanup(() => obj.remove());
+  obj.setAttribute("data", "/common/blank.html");
+  obj.setAttribute("type", "text/plain");
+  obj.setAttribute("typemustmatch", "");
+  obj.onload = t.step_func_done(() => {
+    assert_not_equals(obj.contentDocument, null, "/common/blank.html should be loaded");
+  });
+  obj.onerror = t.unreached_func();
+  document.body.appendChild(obj);
+}, "object's typemustmatch content attribute should not be supported");
 </script>

webdriver/tests/get_element_attribute/get.py

@@ -57,7 +57,6 @@ def test_normal(session):
     ("img", ["ismap"]),
     ("input", ["autofocus", "checked", "disabled", "formnovalidate", "multiple", "readonly", "required"]),
     ("menuitem", ["checked", "default", "disabled"]),
-    ("object", ["typemustmatch"]),
     ("ol", ["reversed"]),
     ("optgroup", ["disabled"]),
     ("option", ["disabled", "selected"]),