The spec can be found online, but it only really details specifically how to decode various features. You can take a look at JXL-Oxide and libjxl's codebases and get a good idea, and before some idiot comes out and speaks about "muh loc" the decoder and encoder bits of libjxl really aren't that hard to read. libjxl's codebase is very well formatted, files are segregated really well into different aspects of the code stream etc.
it's not ideal, but I would rather be given libjxl's codebase over the av1 spec page. (not to mention that heif is paywalled anyways which means that avif, which is heif + av1 is paywalled too... https://www.iso.org/standard/83650.html)
I'm not sure how the benefit could be considered small, being able to perform lossless compression of JPEGs alone is such a massive benefit. In my testing I do every now and then which consists of ripping thousands of images from various websites (image boards, scraping from websites I visit) I regularly get 20-40% file size savings when transcoding JPEG to JXL. These aren't some small 256x256 icons or cherry picked image sets of 25 pictures people love to test. This is as much as a real world use test one could possibly get for web testing.
Completely ignoring the potential of replacing PNG or WEBP, Completely ignoring actually competing against AVIF. The benefits of JXL when it comes to losslessly saving space for pre-existing images is so massively significant, it's hard to believe that this single feature alone doesn't meet the bar of worth.
videos can have their own "containers" too, for instance in AV1 the stream is stored inside of an obu, which is wrapped in a external container. (such as matroska) if you really wanted to you could (and can) put images into containers too, PNGs in a matroska are actually pretty useful way for transfering PNG sequences.
you can also with a simple mod on a older commit of ffmpeg (the commit that added animated jxl broke this method and I haven't gotten around to fixing it) by simply adding jxl 4cc to mux JXL sequences into MKV.
it's not ideal, but I would rather be given libjxl's codebase over the av1 spec page. (not to mention that heif is paywalled anyways which means that avif, which is heif + av1 is paywalled too... https://www.iso.org/standard/83650.html)