My business is 80% mobile traffic, of which 80% is iOS. It feels a lot like the old IE6 days, only (fortunately) less painful.

Yeah I confirm, I spend so much time fixing bugs for IE11 and Safari mobile.

I recall how much joy I felt when my company's IT retired IE 7 and force-upgraded everyone to IE 8. So much polyfills and crutches that we could get rid of... (It was about the same, to a lesser extent, when moving the minimum requirement up to IE 9. That's the point where I stopped thinking about browser support.)

similar market forces at play - apple users use safari coz it is the default browser so you have to develop for safari first not web first just like ms wanted you to develop for IE first not web first.

It's actually the only browser engine on iOS, all the third party ones are just skins on it with convenience features like syncing with their desktop counterparts.

... For some reason. Because Apple. This is one of the things keeping me from taking them seriously.

Doesn't Safari support JPEG 2000 in exchange? And Safari was the one that introduced the newer picture element for enumerating responsive image choices (though to the dismay of HTML semanticists as HTML rather than CSS) wasn't it?

Not really, Firefox and Chrome supported the picture element well before Safari did. It was part of the nascent HTML5 standard.

I believe what WebKit introduced was the analogous CSS image-set property, which likely influenced the picture's element's development (but image-set still hasn't managed to escape draft-spec territory).

Yeah, if you want to serve the most optimal image for Safari (iOS or desktop) JPEG 2000 is the way to go.

The ideal format would be HEIF but there's not a lot of conversion support out there, Imgix doesn't support it for instance.

You can use a polyfill: libwebpjs.