Yahoo Mail Beta is to meet the requirements of Windows Vista OS. and IE7 which is in the Hundreds of Millions of users. It is very expensive to adapt Yahoo Mail to work with any browser other than IE7. Opera has very few users compared to IE7 and this usually means last on the list to get supported.

Well, first of all, if you are using Opera, that means you are in need of a life. So, go get a life, first. Then try out Yahoo Mail Beta.

It’s a beta test - so they would only be testing more common setups, like compatibility with Internet Explorer or Firefox. When it goes live, you might then get more options, like Opera compatibility