Introduce SubmitEvent for the form 'submit' event

It has a submitter attribute, so this fixes #3195.

As part of this, update requestSubmit() to accept null, so that
requestSubmit(e.submitter) always works. (e.submitter is null for
implicit submission, per the discussion in
#3195 (comment).)

Tests: web-platform-tests/wpt#19562

source

@@ -43226,7 +43226,7 @@ interface <dfn>HTMLFormElement</dfn> : <span>HTMLElement</span> {
   <a href="#dom-form-nameditem">getter</a> (<span>RadioNodeList</span> or <span>Element</span>) (DOMString name);
 
   void <span data-x="dom-form-submit">submit</span>();
-  void <span data-x="dom-form-requestSubmit">requestSubmit</span>(optional <span>HTMLElement</span> submitter);
+  void <span data-x="dom-form-requestSubmit">requestSubmit</span>(optional <span>HTMLElement</span>? submitter = null);
   [<span>CEReactions</span>] void <span data-x="dom-form-reset">reset</span>();
   boolean <span data-x="dom-form-checkValidity">checkValidity</span>();
   boolean <span data-x="dom-form-reportValidity">reportValidity</span>();
@@ -43596,7 +43596,7 @@ interface <dfn>HTMLFormElement</dfn> : <span>HTMLElement</span> {
 
   <ol>
    <li>
-    <p>If <var>submitter</var> was given, then:</p>
+    <p>If <var>submitter</var> is not null, then:</p>
 
     <ol>
      <li><p>If <var>submitter</var> is not a <span data-x="concept-submit-button">submit
@@ -56321,9 +56321,14 @@ fur
       </ol>
      </li>
 
+     <li><p>Let <var>submitterButton</var> be null if <var>submitter</var> is <var>form</var>.
+     Otherwise, let <var>submitterButton</var> be <var>submitter</var>.</p></li>
+
      <li><p>Let <var>continue</var> be the result of <span data-x="concept-event-fire">firing an
-     event</span> named <code data-x="event-submit">submit</code> at <var>form</var>, with the <code
-     data-x="dom-Event-bubbles">bubbles</code> attribute initialized to true and the <code
+     event</span> named <code data-x="event-submit">submit</code> at <var>form</var> using
+     <code>SubmitEvent</code>, with the <code data-x="dom-SubmitEvent-submitter">submitter</code>
+     attribute initialized to <var>submitterButton</var>, the <code
+     data-x="dom-Event-bubbles">bubbles</code> attribute initialized to true, and the <code
      data-x="dom-Event-cancelable">cancelable</code> attribute initialized to true.</p></li>
 
      <li><p>Set <var>form</var>'s <span>firing submission events</span> to false.</p></li>
@@ -57083,6 +57088,33 @@ fur
   not reliably interpretable by computer, as the format is ambiguous (for example, there is no way
   to distinguish a literal newline in a value from the newline at the end of the value).</p>
 
+  <h5>The <code>SubmitEvent</code> interface</h5>
+
+  <pre><code class="idl" data-x="">[Exposed=Window]
+interface <dfn>SubmitEvent</dfn> : <span>Event</span> {
+  constructor(DOMString type, optional <span>SubmitEventInit</span> eventInitDict = {});
+
+  readonly attribute <span>HTMLElement</span>? <span data-x="dom-SubmitEvent-submitter">submitter</span>;
+};
+
+dictionary <dfn>SubmitEventInit</dfn> : <span>EventInit</span> {
+  <span>HTMLElement</span>? submitter = null;
+};</code></pre>
+
+  <dl class="domintro">
+   <dt><var>event</var> . <code data-x="dom-SubmitEvent-submitter">submitter</code></dt>
+   <dd><p>Returns the element representing the <span data-x="concept-submit-button">submit
+   button</span> that triggered the <span>form submission</span>, or null if the submission was
+   not triggered by a button.</p></dd>
+  </dl>
+
+  <div w-nodev>
+
+  <p>The <dfn><code data-x="dom-SubmitEvent-submitter">submitter</code></dfn> attribute must return
+  the value it was initialized to.</p>
+
+  </div>
+
   <h5>The <code>FormDataEvent</code> interface</h5>
 
   <pre><code class="idl" data-x="">[Exposed=Window,