It looks like the days of adding attribute filters and body attributes to styles in media queries just to address the Yahoo! Mail media query bug are finally over.

http://freshinbox.com/blog/yahoo-mail-fixes-media-query-bug-yahoo/

No more attribute selector syntax in media queries!

@media (...) {
  div[class=myclass]{...}
}

Not only does it appear that they've fixed the bug, it seems they're supporting responsive emails (min-width, max-width). There's also the possibility of a new Yahoo! Mail client targetting technique leveraging a quirk in how Yahoo! Mail processes media query declarations.