When to Use CSS IDs and Classes
http://www.cssnewbie.com/when-to-use-css-ids-and-classes/

Glazebrook, Rob L.
CSSnewbie
2008

Abstract:

There are three different ways in CSS you can dictate which elements you want to style. Each way is useful for a specific set of purposes, but by using all three together, you can really harness the cascading power of style sheets. The three methods of describing objects on a page are by their tag name, their ID, or their class.