Instances are associated with stylesheets by adding a processing instruction at the top.
<?xml-stylesheet type="text/css" href="mystyle.css"?>
In XHTML, there is also a special element for this:
<link type="text/css" href="mystyle.css" rel="stylesheet"/>
which is in the 'head' element:
<html> <head><title>My Document</title> <link type="text/css" href="mystyle.css" rel="stylesheet"/> </head> <body>...</body> </html>