WOFF files are OpenType or TrueType fonts, with format-specific compression applied and additional XML metadata added.
[4] Following the submission of WOFF to the World Wide Web Consortium (W3C) by the Mozilla Foundation, Opera Software and Microsoft in April 2010,[5][6] the W3C commented that it expected WOFF to soon become the "single, interoperable [font] format" supported by all browsers.
[13] WOFF is a wrapper containing SFNT-based fonts (TrueType or OpenType) that have been compressed using a WOFF-specific encoding tool so they can be embedded in a Web page.
[15] Since OpenType CFF files (with PostScript glyph outlines) are already compressed, their reduction is typically smaller.
Some servers may require the manual addition of WOFF's MIME type to serve the files correctly.