Good for everybody: saves bandwidth.
That's linking to this stylesheet:
Where "res" is for "resource" and "1" means styleid 1.
The stylesheet is mostly static... it's written in S2, so it has access to the style's properties and such.
For cache control, the Last-Modified field of the document returned is going to be the time of the most recently layer in that style.
(And fixed two s2compile errors in the process of doing this.)