Only Firefox supports it, but if you return a request with a header like this:
Header add Link "<style.css>;rel=stylesheet;media=all"
…that will link to that stylesheet without you having to do it in the HTML. Louis Lazaris digs into it:
[…] the only thing I can think of that could justify use for this in production is as a way to include some Firefox-only CSS, which Eric Meyer mentions as a possibility in an old post on this subject. But it’s not guaranteed to always only work in Firefox, so that’s still a problem.
Do with this what you like, but it’s extremely unlikely that this will have any use in a real project.