The "training mode" bit sounds amazing. Imagine training on the top 1000 websites with this in 3 modes: zhtml, zjs, and zcss. Then make the training output a different standard, that basically encode the peculiarities of the languages. Finally apply compression in the server and in the browser, that would be basically the same as zstd but without having to send the dictionary each time.

It might be small gains, it might be large ones on both transfer size and decompression speed, I'd love to see some tests on this. The best thing is that, if a browser (say Chrome) and a CDN (say Cloudflare) agreed on something like this there would be no need to even to anything on the front-end nor the server side, automatic free benefit for the users.