It is actually possible to use Brotli directly in the web browser... with caveats of course. I believe my 2022 submission to JS1024 [1] is the first ever demonstration of this concept, and I also have a proof-of-concept code for the arbitrary compression (which sadly didn't work for the original size-coding purpose though). The main caveat is that you are effectively limited to the ASCII character, and that it is highly sensitive to the rendering stack for the obvious reason---it no longer seems to function in Firefox right now.

[1] https://js1024.fun/demos/2022/18/readme

[2] https://gist.github.com/lifthrasiir/1c7f9c5a421ad39c1af19a9c...