This will be fun.

Ideally, I would like to remove this for all types except images, but there
is some uncertainty wrt to other media load types and making small steps is
easier than huge strides. Especially when it's risking compatibility.

Release Note Request (optional, but appreciated)
[Why is this notable]: We're limiting support for the obscure HTTP Content-Type of multipart/x-mixed-replace due to some security concerns. Other browsers only support this for images, so this should mostly be fine. However, breakage at the HTTP level might be subtle and elevating this to release notes may help people troubleshoot and report back more easily.
[Affects Firefox for Android]: Same. Also removed.
[Suggested wording]: Firefox limits support for the HTTP Content-Type multipart/x-mixed-replace to align with other browsers. Users or enterprises that require ongoing support may change the network.multipart-mixed-replace.enabled_for_document pref to true.
[Links (documentation, blog post, etc)]:

Release Note Request in comment 9 above.

Backed out for causing reftest failures on webcam-simulacrum.mjpg

Backout link

Push with failures

Failure log // TV failure log

Ah, a failing reftest. And for some reasons docshell/test/mochitest/test_bug1747033.html fails in test-verification mode. Fixed the former, gotta further investigate the latter. Thanks for backing out!

Going for another try run https://treeherder.mozilla.org/jobs?repo=try&revision=819eccb75896827365406748baa5d306462a95c8

OK, the change turns test_bug1747033.html into a failure, but only in test-verification mode.
This is interesting because I'm only adding a pref with pushPrefEnv which should be disabled after the test. Effectively disabled/enabled back and forth for every test restart. Not sure what's going on here.

Often tests will already be failing with TV mode and any change (whitespace) will make them fail in TV (because you need to make some change for them to get run by TV).

I suppose the patch here may not be able to land as written. We will still need to support x-mixed-replace on top-level navigations to ImageDocuments. The ContentPolicyType is not a great deciding factor to use.

I suppose we may have to handle x-mixed-replace somehow in Document / ImageDocument loading instead?

Perhaps interpret all mixed replace as images? If it's not an image it'll just display an error suggesting it's an invalid image. Not great failure mode though.

Not sure I can finish this, unfortunately.

Please re-nominate this when it's ready to land.

Thanks tnikkel! It looks like my attempt to force rendering into ImageDocument like you suggested is looking better on try at https://treeherder.mozilla.org/jobs?repo=try&selectedTaskRun=TLiKbuHiT0-O29xALNgg7w.0&revision=debcb89b2fd4ccbac23055a661ef2818136087a8.

I don't have a lot of time to pursue this, so if that fails I'm not sure I can continue with this...

Looks like bugzilla relies on multipart-replace and other browsers still support this terrible ugly wart.
Too bad!

https://bugzilla.mozilla.org/show_bug.cgi?id=1903210

The leave-open keyword is there and there is no activity for 6 months.
:hsinyi, maybe it's time to close this bug?
For more information, please visit BugBot documentation.

I want to give this another try.

The standard still describes multipart document loads. But Chrome restricted this to images only, like here.
I think that's something we ought to track as well.

I'll open a bug for the HTML spec once I am confident that this here works.