Make a.ping and tableCell.headers strings, not DOMTokenLists

Fixes #1026. All current browser implement these as strings, not token
lists; the spec was aspirational in this regard, albeit with vendor
support. However, as discussed in
#1026 (comment),
changing a.ping is not likely to be web-compatible, and changing
tableCell.headers becomes much less interesting if we're not going to be
able to uniformly say that DOMTokenLists are used for token-containing
attributes.

source

@@ -7281,9 +7281,9 @@ a.setAttribute('href', 'http://example.com/'); // change the content attribute d
   the content attribute must be removed, and otherwise, the content attribute must be set to the
   specified new value.</p>
 
-  <p>If a reflecting IDL attribute is a <code data-x="idl-DOMString">DOMString</code> attribute but doesn't fall into any
-  of the above categories, then the getting and setting must be done in a transparent,
-  case-preserving manner.</p>
+  <p>If a reflecting IDL attribute is a <code data-x="idl-DOMString">DOMString</code> or <code
+  data-x="idl-USVString">USVString</code> attribute but doesn't fall into any of the above
+  categories, then the getting and setting must be done in a transparent, case-preserving manner.</p>
 
   <p>If a reflecting IDL attribute is a <code data-x="idl-boolean">boolean</code> attribute, then on
   getting the IDL attribute must return true if the content attribute is set, and false if it is
@@ -18137,7 +18137,7 @@ included with Exhibit B.
 <pre class="idl">interface <dfn>HTMLAnchorElement</dfn> : <span>HTMLElement</span> {
   [<span>CEReactions</span>] attribute DOMString <span data-x="dom-a-target">target</span>;
   [<span>CEReactions</span>] attribute DOMString <span data-x="dom-a-download">download</span>;
-  [<span>CEReactions</span>, SameObject, PutForwards=<span data-x="dom-DOMTokenList-value">value</span>] readonly attribute <span>DOMTokenList</span> <span data-x="dom-a-ping">ping</span>;
+  [<span>CEReactions</span>] attribute USVString <span data-x="dom-a-ping">ping</span>;
   [<span>CEReactions</span>] attribute DOMString <span data-x="dom-a-rel">rel</span>;
   [<span>CEReactions</span>, SameObject, PutForwards=<span data-x="dom-DOMTokenList-value">value</span>] readonly attribute <span>DOMTokenList</span> <span data-x="dom-a-relList">relList</span>;
   [<span>CEReactions</span>] attribute DOMString <span data-x="dom-a-hreflang">hreflang</span>;
@@ -38555,7 +38555,7 @@ the cell that corresponds to the values of the two dice.
     <pre class="idl">interface <dfn>HTMLTableCellElement</dfn> : <span>HTMLElement</span> {
   [<span>CEReactions</span>] attribute unsigned long <span data-x="dom-tdth-colSpan">colSpan</span>;
   [<span>CEReactions</span>] attribute unsigned long <span data-x="dom-tdth-rowSpan">rowSpan</span>;
-  [<span>CEReactions</span>, SameObject, PutForwards=<span data-x="dom-DOMTokenList-value">value</span>] readonly attribute <span>DOMTokenList</span> <span data-x="dom-tdth-headers">headers</span>;
+  [<span>CEReactions</span>] attribute DOMString <span data-x="dom-tdth-headers">headers</span>;
   readonly attribute long <span data-x="dom-tdth-cellIndex">cellIndex</span>;
 
   [<span>CEReactions</span>] attribute DOMString <span data-x="dom-th-scope">scope</span>; // only conforming for th elements