CssRegistration does a grand job of referencing the CSS file relative to the site URL you are on.
What do you do inside the CSS file? If I'm deploying assets to the /_layouts/images directory, sure /_layouts/images works great on MOST sites. There are, however, the occasional /sites/ as a rootweb, in which case the reference needs to be /sites/_layouts/images.
Is there a good way to do this?