Define an optional value and use it for setting dialog's returnValue

Define a new 'optional value' for buttons. This is used instead of 'value' when setting a dialog's returnValue.

Tests:

- web-platform-tests/wpt#51484
- web-platform-tests/wpt#52278

Fixes whatwg#11092.

Author: lukewarlow

source

@@ -51413,6 +51413,10 @@ ldh-str       = &lt; as defined in <a href="https://www.rfc-editor.org/rfc/rfc10
 
   <div w-nodev>
 
+  <p>The element's <span data-x="concept-fe-optional-value">optional value</span> is the value of
+  the element's <code data-x="attr-button-value">value</code> attribute, if there is one; otherwise
+  null.</p>
+
   <p>The element's <span>input activation behavior</span> given <var>event</var> is as follows:</p>
 
   <ol>
@@ -53868,8 +53872,10 @@ interface <dfn interface>HTMLButtonElement</dfn> : <span>HTMLElement</span> {
   <p>The <dfn element-attr for="button"><code data-x="attr-button-value">value</code></dfn>
   attribute gives the element's value for the purposes of form submission. The element's <span
   data-x="concept-fe-value">value</span> is the value of the element's <code
-  data-x="attr-button-value">value</code> attribute, if there is one, or the empty string
-  otherwise.</p>
+  data-x="attr-button-value">value</code> attribute, if there is one; otherwise the empty string.
+  The element's <span data-x="concept-fe-optional-value">optional value</span> is the value of the
+  element's <code data-x="attr-button-value">value</code> attribute, if there is one; otherwise
+  null.</p>
 
   <p class="note">A button (and its value) is only included in the form submission if the button
   itself was used to initiate the form submission.</p>
@@ -56651,6 +56657,11 @@ interface <dfn interface>HTMLLegendElement</dfn> : <span>HTMLElement</span> {
   default value. If it is false, <span data-x="concept-fe-value">value</span> mirrors the default
   value. If it is true, the default value is ignored.</p>
 
+  <p>Some form controls also have an <dfn data-x="concept-fe-optional-value">optional value</dfn>
+  this largely mirrors the <span data-x="concept-fe-value">value</span> but doesn't normalize to an
+  empty string. <span class="note">This ought to be used sparingly, you generally want <span
+  data-x="concept-fe-value">value</span></span>.</p>
+
   <p><code>input</code>, <code>textarea</code>, and <code>select</code> elements have a
   <dfn>user validity</dfn> boolean. It is initially set to false.</p>
 
@@ -60399,8 +60410,9 @@ fur
       </ol>
      </li>
 
-     <li><p>Otherwise, if <var>submitter</var> has a <span data-x="concept-fe-value">value</span>,
-     then set <var>result</var> to that <span data-x="concept-fe-value">value</span>.</p></li>
+     <li><p>Otherwise, if <var>submitter</var> is a <span data-x="concept-submit-button">submit
+     button</span>, then set <var>result</var> to <var>submitter</var>'s <span
+     data-x="concept-fe-optional-value">optional value</span>.</p></li>
 
      <li><p><span>Close the dialog</span> <var>subject</var> with <var>result</var>.</p></li>
 
@@ -62607,7 +62619,7 @@ interface <dfn interface>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
 
     <ol>
      <li><p>Let <var>value</var> be <var>invoker</var>'s <span
-     data-x="concept-fe-value">value</span>.</p></li>
+     data-x="concept-fe-optional-value">optional value</span>.</p></li>
 
      <li><p><span>Close the dialog</span> <var>element</var> with <var>value</var>.</p></li>
     </ol>