That is a huge topic (two big topics, actually).

First, please be aware that CSS and XML are two very different things.  CSS defines the appearance of web page elements.  XML defines a way to structure data (any data, not just web-related data).

A few starting places for CSS:

• http://www.w3schools.com/css/
• http://www.w3.org/Style/CSS/learning
• http://ocw.usu.edu/Instructional_Technology/producing-distance-education-resources/resource90.html

XML is a far broader topic than CSS -- there are many things you can use XML for so you'll want to try to focus on finding XML information for your intended use.  For website design, you might focus on XHTML or XML/XSLT.  For web application design, you'll lean more towards various libraries/methods to encode and parse information in XML format (XSLT is one method).  There are many web-related technologies that use XML but do not require you to know XML in order to use the technology (AJAX, for example).
 
Since you've posted within the SKINNING forum, I'll assume you want to learn CSS for skinning purposes.  As for XML, are you trying to pick apart skin object definitions or is there some other use you're considering?
 
Please respond with exactly what you're wanting to do with CSS and with XML and people will be better able to give you good tips on where to start.