[dom] "A DOMSettableTokenList object is equivalent to ..."

cdumez has just created a new issue for https://github.com/whatwg/dom:

== "A DOMSettableTokenList object is equivalent to ..." ==
https://dom.spec.whatwg.org/#interface-domsettabletokenlist

> A DOMSettableTokenList object is equivalent to a DOMTokenList object
 without an associated attribute.

The HTML specification is using the DOMSettableTokenList in places 
where there is an associated attribute. For e.g. 
HTMLAnchorElement.ping [1]:
[PutForwards=value] readonly attribute DOMSettableTokenList ping;

It needs to be a DOMSettableTokenList because PutForwards needs to be 
able to set the 'value' attribute on the DOMSettableTokenList.

[1] 
https://html.spec.whatwg.org/multipage/semantics.html#the-a-element

See https://github.com/whatwg/dom/issues/81