<img usemap="#map1"> <map name="MAP1">...</map> won't work.
"Compatibility caseless" matching is very complicated, and there were no browsers implementing it correctly. UseCounter proved such complicated algorithm was unnecessary, and even ASCII case-insensitive matching is unnecessary.
Firefox: No public signals
Edge: No public signals
Safari: No public signals
Web developers: No signals
Blink will be the first engine to implement the new behavior. However, web-platform-tests already has automated tests for the new behavior.
According to UseCounters, 0.0002%  +  of page views will break.
Ongoing technical constraints
Will this feature be supported on all six Blink platforms (Windows, Mac, Linux,
Chrome OS, Android, and Android WebView)?
OWP launch tracking bug
Link to entry on the Chrome Platform Status
Requesting approval to ship?
Software Engineer, Google