Part of the CSS selector spec requires that user agents support multiple style classes on any element. This is handled by having a space delimitated list of style classes in the styleClass string. Example from CSS Spec:

For example, the following rule matches any P element whose "class" attribute has been assigned a list of space-separated values that includes "pastoral" and "marine":
p.marine.pastoral { color: green }
This rule matches when class="pastoral blue aqua marine" but does not match for class="pastoral blue".
MORE: http://www.w3.org/TR/CSS2/selector.html#class-html