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But isn't HTTP/2 push restricted to same-origin content? Critical resources may not be same-origin, so I'd expect this technique to have some utility even when HTTP/2 is fully deployed on clients and servers and fully utilized.

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Actually, you're both right. With http/2 the server can push critical assets delivering similar results. But, that requires that the server supports http/2 and is smart enough to initiate the push, AND those resources are same-origin.

The benefit of above technique is that it's deployable today, doesn't require the destination server to be upgraded, and works for cross-origin resources.

Yeah, it would be quite a feat for an http server to force another http server to inject content into it's TCP stream :P (aka, cross origin)

Cross-origin could be an HTTP server itself serving content from a different origin via a push channel (which it could do either because the different origins share servers -- which can occur -- or by having, e.g., prefetched the data itself from the foreign server and pushing it.)

HTTP/2, I'm pretty sure, doesn't allow this (and there are all kinds of reasons it wouldn't be a good idea), but it wouldn't necessarily require forcing the other server to push the content.

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