Re: [dom] DOMTokenList hook to validate added token is in a supported set of keywords (#103)

So if pages start to use
if (link.relList && link.relList.add("preload")) {
that breaks preload on browsers which don't have TokenList implementation updated.
(because .add returning void is false)

This feature feels odd "feature testing" and "actual feature usage" merged together.
Not much different to node.isSupported("foo") + using the feature, except that .add() forces one to actually
try to use the feature in order to know whether browser claims to support it.

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