Brotli is a better example, as it was literally purpose made for HTML/CSS/JS. It is now supported basically everywhere for HTTP compression, and uses a huge custom dictionary (about 120KB) that was trained on simulated web traffic.
You can even swap in your own shared dictionary with a HTTP header, but trying to make your own dictionary specific to your content is a fool’s errand, you’ll never amortize the cost in total bits.
What you CAN do with shared dictionaries though, is delta updates.
