property_accepts_type() only looks at the property itself, not any
longhands it might have, so we thought that `font` didn't accept
`<custom-ident>`s, and seeing "-apple-..." makes us throw out the
declaration even though it's valid as a font name.
We'll reject these like any other unrecognized value if it's somewhere
that's not supported, so this was really just an optimization for a
rare edge case, and removing the check doesn't have any observable
effect except fixing this bug and any similar cases.
Changing property_accepts_type() to look at longhands is not
straightforward, as not all longhand values are valid in the shorthand.
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