WOFF
font/woff
The first real web font format: a TrueType or OpenType font with each table zlib-compressed and a wrapper that browsers agreed to trust cross-origin. Send font/woff, not the application/font-woff label that predates the font top-level type — RFC 8081 created font/* in 2017 precisely to end that guessing game. New projects should skip straight to WOFF2, which compresses better; WOFF 1.0 survives mainly in decade-old @font-face blocks.
Magic bytes
77 4f 46 46 · "wOFF" · offset 0
Per W3C WOFF File Format 1.0 — WOFF header · test a file against it.
Defined by RFC 8081 · IANA registration. Registry facts from the IANA media-types registry via mime-db.