So you may think everything is all-good-thumbs-up when you write .foo div because there are only a few .foos, but actually you are asking the browser to look up all the tens of millions of divs first, then all the .foos and exponentially more with more descendant selectors. Yipes. For more, check out the CSS Tricks article about efficiently rendering CSS.

From : Trello’s blog : refining the way we structure our CSS

The -Must Read- CSS-Tricks article mentionned : CSS-Tricks : Efficiently rendering CSS