Add img controls + alt conformance criteria

Fixes #12377.

Author: yoelhawa

source

@@ -31202,7 +31202,9 @@ interface <dfn interface>HTMLImageElement</dfn> : <span>HTMLElement</span> {
 
   <p>The <dfn element-attr for="img"><code data-x="attr-img-controls">controls</code></dfn>
   attribute is a <span>boolean attribute</span>. If present, it indicates that the user agent may
-  expose a user interface to the user.</p>
+  expose a user interface to the user. The attribute must not be specified on an element that does
+  not have an <code data-x="attr-img-alt">alt</code> attribute, or whose <code
+  data-x="attr-img-alt">alt</code> attribute's value is the empty string.</p>
 
   <div w-nodev>