HTML has a link element to link to external style sheets, but not every XML-based format will have such an element. If there is no suitable element, you can still attach external style sheets by means of the xml-stylesheet processing instruction like this:
<?xml-stylesheet href="my-style.css"?>
... rest of document here...
This processing instruction (PI) must come before the first tag of the document. The type="text/css" is not required, but it helps the browser: if it doesn't support CSS, it knows it won't have to download this file.
Just as with the link element of HTML, there can be multiple xml-stylesheet PIs and they can have attributes to set the type, medium and title.