Oh you're right, Safari does show it. If I go to the URL directly it downloads the file, which is why I assumed it couldn't view it. But it does show on the page https://jpegxl.info/test-page/
Safari was an early adopter of JPEG XL. In the past couple years, actually, the team at Apple responsible for Safari has been making inroads on features and spec work. Jen Simmons especially has been astounding, particularly with her engagement with the community.
She was recently on the Syntax Podcast, and I thoroughly enjoyed the talk. Can catch a bit here: https://www.threads.net/@syntax_fm/post/C22hyslOABy/