Re: [dom] Remove feature detection from add() and add supports() (#123)

> @@ -8555,7 +8556,7 @@ associated <a>attribute</a>'s <a for=Attr>local name</a>.
>  
>  <ol>
>   <li><p>If the associated <a>attribute</a>'s <a for=Attr>local name</a> does not define
> - <a>supported tokens</a>, return true.
> + <a>supported tokens</a>, throw {{TypeError}}.

This should be made consistent.

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