<form> submission checks

Simple tests for web-platform-tests#3109 and whatwg/html#2615.

html/semantics/forms/form-submission-0/submission-checks.window.js

@@ -0,0 +1,62 @@
+async_test(t => {
+  const frame = document.createElement("frame"),
+        form = document.createElement("form");
+  t.add_cleanup(() => frame.remove());
+  form.action = "/common/blank.html";
+  form.target = "doesnotmattertwobits";
+  frame.name = "doesnotmattertwobits";
+  document.body.appendChild(frame);
+  frame.onload = t.step_func(() => {
+    if(frame.contentWindow.location.href === "about:blank")
+      return;
+    assert_unreached();
+  });
+  form.submit();
+  t.step_timeout(() => {
+    assert_equals(frame.contentWindow.location.href, "about:blank");
+    t.done();
+  }, 500);
+}, "<form> not connected to a document cannot navigate");
+
+async_test(t => {
+  const frame = document.createElement("frame"),
+        form = document.createElement("form");
+  t.add_cleanup(() => frame.remove());
+  form.action = "/common/blank.html";
+  form.target = "doesnotmattertwoqbits";
+  form.onsubmit = t.step_func(() => form.remove());
+  frame.name = "doesnotmattertwoqbits";
+  document.body.appendChild(frame);
+  document.body.appendChild(form);
+  frame.onload = t.step_func(() => {
+    if(frame.contentWindow.location.href === "about:blank")
+      return;
+    assert_unreached();
+  });
+  const submit = form.appendChild(document.createElement("input"));
+  submit.type = "submit"
+  submit.click();
+  t.step_timeout(() => {
+    assert_equals(frame.contentWindow.location.href, "about:blank");
+    t.done();
+  }, 500);
+}, "<form> not connected to a document after submit event cannot navigate");
+
+async_test(t => {
+  const frame = document.createElement("frame"),
+        form = document.createElement("form");
+  t.add_cleanup(() => frame.remove());
+  form.action = "/";
+  document.body.appendChild(frame);
+  frame.contentDocument.body.appendChild(form);
+  frame.onload = t.step_func(() => {
+    if(frame.contentWindow.location.href === "about:blank")
+      return;
+    form.submit();
+    t.step_timeout(() => {
+      assert_equals(frame.contentWindow.location.pathname, "/common/blank.html");
+      t.done();
+    }, 500)
+  });
+  frame.src = "/common/blank.html";
+}, "<form> in a navigated document cannot navigate");