Ported from https://www.w3.org/Bugs/Public/show_bug.cgi?id=24055. Standards like Notifications or Web App Manifest need to fetch images. It's unclear which portions of <img>'s very-complicated processing model they should involve, but in that bug people seem to be advocating for at least MIME sniffing and the "list of available images" cache.

This also impacts <input type="image">, and probably <link rel="icon">.

The thread is long and someone planning to tackle this should probably read through it and distill it here before going too far.

Note: #1643 is probably a prerequisite for this issue.