Yahoo! mail, specifically in Firefox and Chrome, ignores your media query declaration along with all of its conditional statements. It then renders each of the styles as if they are outside of the media query to begin with. This can become problematic when you are developing emails that have a logo in your footer, and you want to position it based on desktop or mobile view. See the image below as an example:

In this situation, the logo underneath the hyperlinks should be hidden, only appearing on mobile email clients. Here is the initial HTML.

HTML

<a href="http://www.website.com" target="_blank" style="text-decoration:none;color:#ffffff;">
   <img src="logo.png" alt="logo" width="116" height="31" class="show-mobile" style="display:none; border:none; border-style:none;"/>
</a>

The inline style of display:none; and class=”show-mobile” are in bold. The former is intended to hide the logo under the links in desktop view, while the latter is meant to display the logo in mobile view.

Below is the source code that is generated by the Yahoo! email client.

Yahoo! mail will prepend the following string “yiv##########” to your inline css that is used to show the element in mobile devices, as you can see it does so with class=”show-mobile”.

Despite the fact that there is now a Yahoo! media query that will fix this bug, attribute selectors are still not supported. Using a CSS rule as shown below, will not work when trying to target the Yahoo! generated CSS class.

CSS

[class^="yiv"][class$=”show-mobile”] {
   display: none;
} 

Just for kicks, even if you explicitly create a rule for the generated class, it will still not work:

CSS

[class=”yiv0779069323show-mobile”]
   display:none;
}

Using the Yahoo! media query will do the trick and hide the logo.

CSS

@media screen yahoo {
   span[class="show-mobile"] {
      display: none !important;
   }
}

See the intended result below.

Please note that I discovered that this problem only happens in Firefox and Chrome only, not in IE.

The Email on Acid website has two terrific articles on their website that were of great help when trying to solve this problem, https://www.emailonacid.com/blog/article/email-development/9_ways_to_prevent_yahoo_headaches#yahoo_tip8 and https://www.emailonacid.com/blog/article/industry-news/yahoo_mail_now_supports_media_queries.