So quite a few frameworks are becoming mobile in how they handle CSS. Responsive design has been taking a mobile first view for a while now, but mobile first CSS at first, to me, seemed a bit strange of a stance to take for web development, namely because of browsers that do not support media queries. The browsers in particular that becomes a worry with approach, is Opera 9 and under, Safari 3.2 and under, Firefox 3 and under, and IE8 and under. In practical terms however, the only ones we really have to concern ourselves with are IE8 and below, since Opera and Firefox users would likely have long been nagged to upgrade their browsers to a more recent release.
Under these circumstances, I've started to become more favorable to mobile first CSS, since if you are dealing with IE versions older than IE9, particularly IE7 and IE6, quite a large part of your modern CSS principles will not work, and if you are building for progressive enhancement, you're main CSS will likely be bogged down with Fallbacks, which with mobile first CSS we can isolate to only IE specific style sheets. Additionally, this gives us the added benefit of allowing our responsive design to function on mobile devices that do not support media queries, though how much of a benefit that is remains to be seen, especially now that the mobile web is largely dependent on touch interaction in its UI patterns. Most mobile websites would struggle with a tab interface like an old Blackberry.
There is also the use of CSS preprocessors like SASS and LESS (SASS being my preferred tool) which will easily allow for compartmentalization of sections of your CSS, which comes in handy for porting sections of code around for various stylesheets or fallback stylesheets. Honestly, I wish that one of my current projects had been built CSS first, as it really would have saved quite a bit of effort in the code, and would have allowed me to more easily support outdated browsers.