Everything you need to know about WOFF
WOFF (Web Open Font Format, .woff) is a wrapper format for TrueType and OpenType fonts, created by Mozilla in 2009 specifically for web delivery. It compresses font data (~40% smaller than raw TTF) and prevents desktop installation - addressing the licensing concerns that kept foundries from licensing fonts for the web.
How it works under the hood
- Wraps TTF/OTF. WOFF doesn't replace TTF/OTF - it wraps them with a custom compression layer (zlib/DEFLATE).
- Zlib compression. WOFF uses standard DEFLATE - same as ZIP and PNG. Decompression is fast on every modern device.
- Metadata block. WOFF has a dedicated metadata XML block where foundries can include licensing info, designer credits, and copyright.
- Browser-only. Operating systems generally won't install WOFF as a system font - this enforces the 'web only' license intent.
Where you'll actually use it
- Web font delivery before WOFF2 became universal (2016)
- Fallback for IE9-11 (which support WOFF but not WOFF2)
- Older Android Browser and legacy mobile browsers
- Email signatures (rare but real)
How it compares to alternatives
WOFF vs TTF: WOFF is ~40% smaller and web-only. WOFF vs WOFF2: WOFF2 uses Brotli compression - 30% smaller again. WOFF was the bridge from TTF to WOFF2.
Things that will trip you up
- Always serve WOFF2 first, WOFF as fallback - no browser supports WOFF without WOFF2 except very old IE
- Some `font-display` strategies cause FOUC (flash of unstyled content) - test with throttled network
- Hosting WOFF cross-origin requires CORS headers - missing CORS = silent font failure in Firefox
Test it yourself: Browser DevTools Network tab shows font requests, `woff2_decompress` (brotli tool) for inspection. WOFF Toolkit for command-line conversion.