[heinrich5991]

This also dramatically increases your fingerprintability, I think. Fingerprinters probably aren't using it yet though, because it's such a small target audience. But if it were to be implemented into a major browser, I'm sure they'd start using it.

[Retr0id]

To elaborate, JS running on a page can probe your cache (via timing) to discern which URLs are already in it or not. If the cache is shared cross-origin, then it can be used to leak information cross-origin.