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Nothing seems to make "data://" a parse error.
That is deferred to the "Fetch layer". See bug 20045. In the URL parser anything goes.
Does this match implementations? Whether an error occurs at URL parse/resolve time or at fetch time affects e.g. navigation.
As per usual, the answer is in the middle. Opera and Chrome navigate. Firefox (does not even make it a link) and Safari (makes it a link, but clicking it does nothing) do not. I prefer not inlining the data URL parser, but I can ask around a bit more first.
So long as we're not going against interop, I'm happy with any solution here.
Lets leave it like this for now.