One of the lesser known quirks of Yahoo! Mail involves their CSS parser removing the next CSS style after comments. In this article, we'll go into how to address this issue by adding a phantom selector under comments as well as using this to your benefit by using it as a means to target Yahoo! Mail.
Strangely however, the comment bug only affects the Webmail version and not the Yahoo! Mail app...
FreshInbox
Interactive and Advanced Email Design Techniques and News