Empty <img> is now spec-non-compliant

[classilla]

https://bugzilla.mozilla.org/show_bug.cgi?id=1196668
https://bugzilla.mozilla.org/show_bug.cgi?id=1506592
https://bugzilla.mozilla.org/show_bug.cgi?id=1507114 ???
https://bugzilla.mozilla.org/show_bug.cgi?id=1616537

See also PM 1543

[classilla]

We don't need M1507114.

[classilla]

Tested to match Fx75.