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/
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/