<a>, <area>, and <link> must have an active document check (#5758)

html/semantics/links/following-hyperlinks/active-document.window.js

@@ -0,0 +1,23 @@
+["a",
+ "area",
+ "link"].forEach(type => {
+  async_test(t => {
+    const frame = document.createElement("iframe"),
+          link = document.createElement(type);
+    t.add_cleanup(() => frame.remove());
+    frame.onload = t.step_func(() => {
+      // See https://github.com/whatwg/html/issues/490
+      if(frame.contentWindow.location.href === "about:blank")
+        return;
+      link.click(); // must be ignored because document is not active
+      t.step_timeout(() => {
+        assert_equals(frame.contentWindow.location.pathname, "/common/blank.html");
+        t.done();
+      }, 500);
+    });
+    document.body.appendChild(frame);
+    frame.contentDocument.body.appendChild(link);
+    link.href = "/";
+    frame.src = "/common/blank.html";
+  }, "<" + type + "> in navigated away <iframe>'s document cannot follow hyperlinks");
+});