sometimes it's useful to check for the existence of two classes in an element before applying a style to it. For example, if you have a front page that has a different width sidebar than an interior page, you could use a separate page-front.tpl.php file, kind of overkill of your just changing the width of the sidebar.
Another option is to use what http://www.bennadel.com/blog/680-Defining-A-CSS-Selector-That-Requires-A... calls multi class selectors.
.front.sidebar-left #content{
width:760px;
margin-left:436px;
margin-right:-1196px;
