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Well, there are de jure standards (what the w3c says a browser should do) and de facto standards (what Chrome does).

As it ever was. Standards are a three-edged sword: spec, intent of spec, and implementations of spec.

Which standard requires support of JXL?

The PDF association apparently recently added jpeg xl to the pdf spec and indicated that it's the preferred solution for HDR content.

Then again PDF also technically supports embedded audio, video, 3D graphics, and arbitrary Javascript. If Flash hadn't died it would probably still support that too. It's a clown car format where everyone besides Adobe just tacitly agrees to ignore huge chunks of the spec.

> It's a clown car format

As is the destiny of any document format in wide spread use, PDF had flash, doc had ActiveX.

Also this text is formatted using a mark down language fully capable of embedding entire applications.

PDF had Flash support? I thought the Flash Xtra for Shockwave was nuts...

Web standard I meant. The OP didn't talk about PDFs from context.

Obligatory xkcd: https://xkcd.com/927