Explicitly state “is” attribute conformance requirements

source

@@ -65843,10 +65843,11 @@ console.log(plasticButton2.getAttribute("is")); // will output "plastic-button"<
    <span data-x="concept-custom-element-definition-name">defined name</span>.</p></li>
 
    <li><p>A <dfn data-export="">customized built-in element</dfn>, which is defined with an <code
-   data-x="">extends</code> option. These types of custom elements have local name equal to the
+   data-x="">extends</code> option. These types of custom elements have a local name equal to the
    value passed in their <code data-x="">extends</code> option, and their <span
    data-x="concept-custom-element-definition-name">defined name</span> is used as the value of the
-   <dfn><code data-x="attr-is">is</code></dfn> attribute.</p></li>
+   <dfn><code data-x="attr-is">is</code></dfn> attribute, which therefore must be a <span>valid
+   custom element name</span>.</p></li>
 
   </ol>