Introduce enumerated attribute reflection IDL

- [ReflectEnum=identifier] can be used alongside [Reflect] on DOMString and DOMString? attributes to identify the IDL enum which holds the known value keywords.

Additionally, 3 new extended attributes are allowed on enums:

- [InvalidValueDefault] and [InvalidValueDefault=string] sets the value to be used when the content attribute has an invalid value.

- [MissingValueDefault] and [MissingValueDefault=string] sets the value to be used when the content attribute is missing.

- [EmptyValueDefault] and [EmptyValueDefault=string] sets the value to be used when the content attribute is the empty string.

Author: lukewarlow

source

@@ -8816,9 +8816,11 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
   <dfn extended-attribute data-lt="ReflectPositiveWithFallback"
   data-x="xattr-ReflectPositiveWithFallback"><code>[ReflectPositiveWithFallback]</code></dfn>, <dfn
   extended-attribute data-lt="ReflectRange"
-  data-x="xattr-ReflectRange"><code>[ReflectRange]</code></dfn>, and <dfn extended-attribute
-  data-lt="ReflectDefault" data-x="xattr-ReflectDefault"><code>[ReflectDefault]</code></dfn> IDL
-  <span>extended attribute</span>s.</p>
+  data-x="xattr-ReflectRange"><code>[ReflectRange]</code></dfn>, <dfn extended-attribute
+  data-lt="ReflectDefault" data-x="xattr-ReflectDefault"><code>[ReflectDefault]</code></dfn>, and
+  <dfn extended-attribute data-lt="ReflectEnum"
+  data-x="xattr-ReflectEnum"><code>[ReflectEnum]</code></dfn> IDL <span>extended
+  attribute</span>s.</p>
 
   <p>The <code data-x="xattr-Reflect">[Reflect]</code>, <code
   data-x="xattr-ReflectURL">[ReflectURL]</code> <code
@@ -8889,6 +8891,33 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
   <span>extended attribute</span> have a <span>default value</span> provided by <code
   data-x="xattr-ReflectDefault">[ReflectDefault]</code>.</p>
 
+  <p>The <code data-x="xattr-ReflectEnum">[ReflectEnum]</code> <span>extended attribute</span> must
+  take an identifier. It must not appear on anything other than an interface member attribute with a
+  type of <code data-x="idl-DOMString">DOMString</code>. Additionally, it must also only appear
+  alongside <code data-x="xattr-Reflect">[Reflect]</code>. The identifier provided by <code
+  data-x="xattr-ReflectEnum">[ReflectEnum]</code> must be that of an IDL enum, this is used to
+  represent the known-value keywords.
+
+  <p>To declare the value used for the <span>invalid value default</span>, <span>missing value
+  default</span>, and the empty value states, we introduce the <dfn extended-attribute
+  data-x="xattr-InvalidValueDefault"><code>[InvalidValueDefault]</code></dfn>, <dfn
+  extended-attribute data-x="xattr-MissingValueDefault"><code>[MissingValueDefault]</code></dfn>,
+  and <dfn extended-attribute
+  data-x="xattr-EmptyValueDefault"><code>[EmptyValueDefault]</code></dfn> IDL <span>extended
+  attribute</span>s.</p>
+
+  <p>The <code data-x="xattr-InvalidValueDefault">[InvalidValueDefault]</code>, <code
+  data-x="xattr-MissingValueDefault">[MissingValueDefault]</code> and <code
+  data-x="xattr-EmptyValueDefault">[EmptyValueDefault]</code> <span>extended attribute</span>s must
+  either take no arguments, or take a string. They must not appear on anything other than an
+  enum.</p>
+
+  <p>IDL attributes with the <code data-x="xattr-ReflectEnum">[ReflectEnum]</code> <span>extended
+  attribute</span> are <span>limited to only known values</span>.</p>
+
+  <p>TODO: Define how [InvalidValueDefault], [MissingValueDefault], and [EmptyValueDefault] actually
+  work.</p>
+
   <h4>Using reflect in specifications</h4>
 
   <p><span data-x="reflect">Reflection</span> is primarily about improving web developer ergonomics
@@ -12034,7 +12063,14 @@ interface <dfn interface>HTMLElement</dfn> : <span>Element</span> {
   undefined <span data-x="dom-showPopover">showPopover</span>(optional <span>ShowPopoverOptions</span> options = {});
   undefined <span data-x="dom-hidePopover">hidePopover</span>();
   boolean <span data-x="dom-togglePopover">togglePopover</span>(optional (<span>TogglePopoverOptions</span> or boolean) options = {});
-  [<span>CEReactions</span>] attribute DOMString? <span data-x="dom-popover">popover</span>;
+  [<span>CEReactions</span>, <span data-x="xattr-Reflect">Reflect</span>, <span data-x="xattr-ReflectEnum">ReflectEnum</span>=<span>PopoverKeywords</span>] attribute DOMString? <dfn attribute for="HTMLElement" data-x="dom-popover">popover</dfn>;
+};
+
+[<span data-x="xattr-InvalidValueDefault">InvalidValueDefault</span>=<span data-x="attr-popover-manual">"manual"</span>, <span data-x="xattr-MissingValueDefault">MissingValueDefault</span>, <span data-x="xattr-EmptyValueDefault">EmptyValueDefault</span>=<span data-x="attr-popover-auto">"auto"</span>]
+enum <dfn enum>PopoverKeywords</dfn> {
+  <dfn attr-value for="html-global/popover" data-x="attr-popover-auto">"auto"</dfn>,
+  <dfn attr-value for="html-global/popover" data-x="attr-popover-manual">"manual"</dfn>,
+  <dfn attr-value for="html-global/popover" data-x="attr-popover-hint">"hint"</dfn>
 };
 
 dictionary <dfn dictionary>ShowPopoverOptions</dfn> {
@@ -86323,23 +86359,20 @@ dictionary <dfn dictionary>DragEventInit</dfn> : <span>MouseEventInit</span> {
      <th>Brief description
    <tbody>
     <tr>
-     <td><dfn attr-value for="html-global/popover"><code
-     data-x="attr-popover-auto">auto</code></dfn>
-     <td rowspan=2><dfn data-x="attr-popover-auto-state">Auto</dfn>
+     <td><code data-x="attr-popover-auto">auto</code></td>
+     <td rowspan=2><dfn data-x="attr-popover-auto-state">Auto</dfn></td>
      <td rowspan=2>Closes other popovers when opened; has <span data-x="popover light dismiss">light
      dismiss</span> and responds to <span data-x="close request">close requests</span>.
     <tr>
      <td>(the empty string)
     <tr>
-     <td><dfn attr-value for="html-global/popover"><code
-     data-x="attr-popover-manual">manual</code></dfn>
-     <td><dfn data-x="attr-popover-manual-state">Manual</dfn>
+     <td><code data-x="attr-popover-manual">manual</code></td>
+     <td><dfn data-x="attr-popover-manual-state">Manual</dfn></td>
      <td>Does not close other popovers; does not <span data-x="popover light dismiss">light
      dismiss</span> or respond to <span data-x="close request">close requests</span>.
     </tr>
     <tr>
-     <td><dfn attr-value for="html-global/popover"><code
-     data-x="attr-popover-hint">hint</code></dfn></td>
+     <td><code data-x="attr-popover-hint">hint</code></td>
      <td><dfn data-x="attr-popover-hint-state">Hint</dfn></td>
      <td>Closes other hint popovers when opened, but not other auto popovers; has <span
      data-x="popover light dismiss">light dismiss</span> and responds to <span data-x="close
@@ -86353,10 +86386,6 @@ dictionary <dfn dictionary>DragEventInit</dfn> : <span>MouseEventInit</span> {
   default">invalid value default</i> is the <span data-x="attr-popover-manual-state">Manual</span>
   state.</p>
 
-  <p>The <dfn attribute for="HTMLElement"><code data-x="dom-popover">popover</code></dfn> IDL
-  attribute must <span>reflect</span> the <span data-x="attr-popover">popover</span> attribute,
-  <span>limited to only known values</span>.</p>
-
   <p>Every <span data-x="HTML elements">HTML element</span> has a <dfn export>popover visibility
   state</dfn>, initially <span data-x="popover-hidden-state">hidden</span>, with these potential
   values:</p>