Gmail's HTML-only mode works fine in Opera 12, and unless you really like the (imho slow and bloated) interface of regular gmail, it is better. The only issues

Gmail's HTML-only mode works fine in Opera 12, and unless you really like the (imho slow and bloated) interface of regular gmail, it is better. The only issues I had were bulk deleting spam and trash, it took me only a 5 line userscript to add a "select-all" button and the problem is solved. Also worth mentioning that this interface isn't exactly html-only, it still has some assisting javascript like automatically completing typed addresses from your contacts.