CSS Processor utility/API?

Hello,

This is the first time I have posted to this group. I looked through the
archives, but I did not find a similar question. 

I was wondering if there is any utility/API that could be used to process
CSS on an XHTML file. Such a utility would need to take as inputs, a CSS
file and an XHTML (or XML) file. Before processing, you would write a
'Handler'. The CSS processor would make call-back functions on the Handler,
similar to the way SAX or SAC do. The call-back method will have parameters,
namely the Nodes selected and the style actions. The Handler will know what
to do with the Nodes and the style actions. 

I guess this is something a browser that supports CSS would do. But I was
wondering if there is any open API (and hopefully an implementation) for
this. Thanks in advance for any pointers or resources.

Regards,
Niranjan