As many other web developers out there probably do something like this, I thought I'd point out that the new CSS feature don't play well with the common use of tiles.
A common approach is to define your boilerplate markup and layout in one jsp:
Your other pages refer to this layout and simply define the named fragments that get inserted into it:
My point is that, using such a system, its unlikely that your css styles will be declared in the jsp as the stylesheet reference. This is common in Struts, and I believe that WebWork has a similar type of templating system.
Not sure what to suggest - any chance you can just assume that all of the CSS files in the project are potential sources of styles if no explicit stylesheet reference exists in the fragment being edited?