[css-forms-1] Add fieldset and legend (#11981)

Author: lukewarlow

css-forms-1/Overview.bs

@@ -93,7 +93,7 @@ spec:css-forms-1; type:value; for:/; text:::placeholder
   To apply the [=basic appearance=] on individual controls, the following code is used:
 
   <pre class="lang-css">
-    input, textarea, meter, progress, button, select {
+    input, textarea, meter, progress, button, select, fieldset, legend {
       appearance: base;
     }
   </pre>
@@ -119,7 +119,7 @@ spec:css-forms-1; type:value; for:/; text:::placeholder
       color: rgb(131, 17, 0);
     }
 
-    input, textarea, meter, progress, button, select {
+    input, textarea, meter, progress, button, select, fieldset, legend {
       appearance: base;
     }
     </pre>
@@ -136,7 +136,7 @@ spec:css-forms-1; type:value; for:/; text:::placeholder
       color: rgb(0, 249, 0);
     }
 
-    input, textarea, meter, progress, button, select {
+    input, textarea, meter, progress, button, select, fieldset, legend {
       appearance: base;
     }
     </pre>
@@ -295,6 +295,10 @@ spec:css-forms-1; type:value; for:/; text:::placeholder
         <td>`<option>`</td>
         <td>''::checkmark''</td>
       </tr>
+      <tr>
+        <td>`<fieldset>` and `<legend>`</td>
+        <td></td>
+      </tr>
       <tr>
         <td>Buttons</td>
         <td></td>
@@ -695,6 +699,14 @@ spec:css-forms-1; type:value; for:/; text:::placeholder
     <span class="fake-textarea normal">The quick brown fox jumps over the lazy dog.&#x23B8;</span>
   </div>
 
+## Fieldset layout ## {#fieldset-layout}
+
+  ISSUE: Move to HTML.
+
+  When a fieldset is in base appearance mode the layout and rendering behaviour as
+  [defined](https://html.spec.whatwg.org/multipage/#the-fieldset-and-legend-elements:the-fieldset-element-2) in the HTML
+  specification, do not apply to the fieldset or its legend.
+
 ## Changing the Orientation of a [=Slider-Like Control=]: ''slider-orientation'' ## {#slider-orientation}
 
   ISSUE: Rework this property.
@@ -819,7 +831,9 @@ button,
 ::file-selector-button,
 select,
 meter,
-progress {
+progress,
+fieldset,
+legend {
     color: inherit;
     font: inherit;
     box-sizing: border-box;
@@ -834,7 +848,8 @@ textarea,
 button,
 ::file-selector-button,
 ::slider-track,
-select {
+select,
+fieldset {
   border: 1px solid currentColor;
   background-color: transparent;
 }