Yahoo! Mail released an update that came with a new UI, a few new features and some performance improvements. Yahoo! Mail also made some tweaks on how they process CSS as well. The significant change is that the new Yahoo! Mail no longer strips display:none styles when applied inline.
Since this release is considered “beta”, there will probably be a sizable amount of users using the old Yahoo! Mail until this release becomes the default.
Read more about the changes on the Email on Acid blog.