I'm currently working on an open-source project. Currently, we put our copyright notice at the top of every Java source file.
I was wondering whether it was advisable or common practice to put copyright notices in CSS or XML files.
CSS yes, XML not so much
The purpose of a copyright notice is to inform readers that the material has a certain level of protection against unauthorised reuse. For creative works, such as CSS styling or source code, this makes sense.
In the case of XML it seems redundant because the XML should be acting as a data carrier rather than a creative work in and of itself. Of course, as Benjamin Gruenbaum has noted in the comments, if the XML is carrying a copyrighted work then that should carry a copyright notice within it.
On the whole, try to keep your copyright notice as small and unobtrusive as possible since people on tight bandwidth limits don't want to download more data than is necessary.