I think Opera Mini did much more than compression.
I seem to remember them actually rewriting (rendering?) the web page server side, and then sending an optimized mobile friendly version to make it more readable for pages that were originally designed for desktop.
It worked great on my phone and I even used it when the phones were more cabable, because web pages looked better and it saved a lot of bandwidth.
They rendered the site server-side (including JavaScript execution for a couple of seconds and all) and then sent the rendered page in some binary markup language to the client, with images heavily compressed.
I actually still sometimes use it on iOS! The app is no longer available in most (all?) App Store regions, but I still have it on my account and can redownload it. The servers seem to still be there!
I seem to remember them actually rewriting (rendering?) the web page server side, and then sending an optimized mobile friendly version to make it more readable for pages that were originally designed for desktop.
It worked great on my phone and I even used it when the phones were more cabable, because web pages looked better and it saved a lot of bandwidth.