JPEG 2000 is an actual standard but to get it working you need to license a decoder from someone or build a new one, you can’t just piggyback on some existing HEVC decoder. JPEG 2000 doesn’t currently work in every browser, and there have been at least a couple of security vulnerabilities in the decoder (Kakadu I think?) that OS X ships by default and enables for Safari. In many cases where there’s no native browser or direct plugin support you can get it working with a slow, buggy, resource-hogging Flash implementation, but I’ve had pretty poor experience as a user on sites that incorporate one. I don’t believe there are any pure-Javascript implementations.
As far as I can tell, this new format already has a better client compatibility story than JPEG 2000 right off the bat, as well as better quality for the same file sizes.
