If you reencode a jpeg to a png, you used a lossless format but your image is very lossy.
But it's also a valid point.
JPEG artifacts are literally a meme, normal people are aware of this stuff.
I hold the extremist view that had we decided to use .png for lossless webps, it would have been an overall net gain - in people's minds "png" maps roughly to "whoever last touched this image didn't do anything evil to it" - very few people care how the data is actually encoded.
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
Like, you can do this individually, but it's not the same thing as actually having it in the standard.
But people are definitely going to assume "eXtra Large". Lots of people. I guess a few will think "excel" (as in JPEG XL comes out ahead and is superior).
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Does the one at the end of .JXL count toward the total? Nobody knows what it stands for anyway.
(You know... for "Wumbo")
Like files.tar.xz may be decompressed with xz and then extracted with tar to get the directory "files".
I think in your case .md.fm would be a greater fit, as the front matter is read first. Or just .fmd