You can use rendered to compute whether a resource is used or not. Here are some examples:
<resource rendered="#{javascript:context.getUserAgent().getUserAgent().match("iPhone")}">
<content-type>text/css</content-type>
<href>iPhone.css</href>
</resource>
<resource rendered="#{javascript:context.getUserAgent().isIE(0,6) == true}">
<content-type>text/css</content-type>
<href>ie6.css</href>
</resource>
So, using the rendered property you can control when to load a specific resource. Does that answer your question?