I would like to suggest we remove support for 'multipart/x-mixed-replace’ main 
resources while keeping support for multipart images.

Based on Chrome usage data, this feature is extremely rarely used by Web sites 
(less than 0.00001% of page loads) [1]. This feature adds complexity to the 
loader and is a source of (security) bugs (e.g. [2] recently), current support 
also seems buggy.

Current support in Safari / WebKit:
- Support is not great is WebKit. If you load a Motion JPEG main resource for 
example, it will keep creating a new ImageDocument and all its DOM tree for 
every frame (tested on Safari / Mac).
- It looks like support is broken on Safari on iOS (I tried a Motion JPEG main 
resource on iOS8, I see the first frame then a blank page that never finishes 

Other browsers:
- Never supported by IE (including IE11) for any resource
- Chrome already dropped support for this (main resources only) almost 2 years 
ago [3].
- Firefox 37 still supports this based on local testing.

Again, I am only proposing dropping support for main resources. For e.g., 
having an <IMG> element in a page whose src attribute points to a Motion JPEG 
would still work as intended.

[1] https://code.google.com/p/chromium/issues/detail?id=249132 
[2] https://bugs.webkit.org/show_bug.cgi?id=143979 
[3] http://src.chromium.org/viewvc/blink?view=revision&revision=152363 

 Chris Dumez - Apple Inc. - Cupertino, CA

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