Perhaps I missed something along the way - but I recently I discovered that you can apply multiple CSS classes to an element and "combine" them.

For example.

.normal_box      { border: 1px solid #CCCCCC; padding: 20px; }
.small_text      { font-family: 9px; }
.red_text        { color: red; }

<div class="normal_box small_text red_text">
  Some small red text!
</div>

It might not immediately seem useful - but it is great when you want to set up some "base" styles and re-use them for multiple elements without repeating the same thing over and over. A nifty little trick is using it for "on" and "off" states for buttons, etc - you can simple have something like class="button on".

Anyway, just thought I would mention it, as I must admit that it is news to me.