You cannot tell it just from the extension either. If you like extensions, you can use a '.lossless.jxl'. And in general for most formats, you can use metadata to store that info (also doesn't guarantee anything)

You can’t really do that with PNG today either, because of all the PNG optimizers that quantize images before encoding them to PNG for better compression.

That's true of WebP and AVIF as well. I think all modern image codecs have both lossy and lossless modes - the separation between JPEG for lossy and PNG for lossless seems to be a historical oddity.

This is indeed frustrating, though realistically not that big of a deal for most users, especially if you consider lossless as just highest quality. Though personally I think it makes sense to have different extensions by convention for lossless and animated images.

ah, that's a good point.

perhaps use .ll.jxl to indicate "lossless jpegxl" informally?

prior art: I've been using .frontmatter.md or .fm.md for markdown files with frontmatter

I can't really think of a scenario where that would be useful? File-size is more indicative of quality anyway.