text/css

What a stylesheet must be labelled before browsers will trust it: in standards mode, a link rel=stylesheet that comes back as text/plain or text/html is silently discarded — one of the few places strict MIME enforcement predates nosniff. The registration (RFC 2318) is old enough to predate CSS 2, but it still holds. Encoding is the easy part: CSS defaults to UTF-8, with an @charset rule available for the stragglers.

Defined by RFC 2318 · IANA registration. Registry facts from the IANA media-types registry via mime-db.