Opera Mini would send binary markup and heavily compressed images [0]. The final rendering and any interactions happened on the user’s device. This service is basically remote desktop (RDP/VNC) to a copy of Chrome running in The Cloud. Opera Mini was also the most popular in a different Internet landscape (no SPAs, smaller sites overall, tiny non-touchscreen dumbphones).
On a related topic, I actually chose my phone around ~2004 based on the full Opera Mobile browser being included (Nokia 3650 I think it was). The ability to bring up and use almost any site and browse around in a 2 inch screen like a little portal into the web was great. It was much much better than any other mobile browser I had used because it was fully rendering instead of converting for mobile (though that may have been an option). Was one step away from what people started to experience with the touch phones that came out a few years later.
[0]: https://en.wikipedia.org/wiki/Opera_Mini#Functionality