Depends on what kind of document it is. Most likely you want an inline stylesheet.

Well, for the record I was wrong about the URL. It's been changed to resource://gre/res/mathml.css .

Inline stylesheets are frowned upon for HTML documents (the <style> element is deprecated), and effectively forbidden in XML documents. The exception is if you include a namespaced <html:style/> element, and that's even more frowned upon.