COOP and multipart/x-mixed-replace

[domenic]

From #5334 (comment):

I noticed that multipart/x-mixed-replace also invokes "process a navigate response" and can do so from a top-level browsing context. Those subsequent responses should never be allowed to change COOP+COEP state I think, but this is not specified now. We might also not have test coverage there, because it's in general a somewhat buggy part of the whole navigation algorithm.

[jakearchibald]

https://random-server-stuff.glitch.me/multipart-mixed-replace - simple text/plain x-mixed-replace example.

It seems like Firefox rejects the above, Chrome tries to parse it as an image, but Safari parses it properly. It's possible I've gotten the protocol wrong though.

If the results are correct, I think spec'ing one of those makes sense.

The Firefox behaviour is simplest, but fails if you navigate directly to one of those old style 'MJPEG' webcam images. If we care about that, Chrome's behaviour is a good compromise. Safari's behaviour is best if there are resources using x-mixed-replace for non-image resources, but I don't think that's the case.

[jakearchibald]

Hm, I guess Chrome's solution doesn't really help here as it still needs to create a same-origin document for the image.