<style>
.navigation.mainPage { color:green; font-weight: bold }
</style>

This is a test case for the SO question CSS classes: base+additional. How to define?. Your browser will show the matching elements like this.

Browsers should ONLY match elements with BOTH classes (see left: Opera 10.00.1750, Win XP SP3). However, IE6 matches anything with the second class (see right: IE 6.0.2900.5512, no browser extensions, Win XP SP3). NOTE: Be aware that IE8 has approximately 29.5 different rendering modes - if your site displays in Quirks Mode, you'll get the (broken) behavior of IE6!

left: Mozilla Firefox 3, right: IE8 with X-UA-Compatible: IE=edge