Why should we include ttf, eot, woff, svg,... in a font-face

Answer in 2019:

Only use WOFF2, or if you need legacy support, WOFF. Do not use any other format

(svg and eot are dead formats, ttf and otf are full system fonts, and should not be used for web purposes)

Original answer from 2012:

In short, font-face is very old, but only recently has been supported by more than IE.

eot is needed for Internet Explorers that are older than IE9 - they invented the spec, but eot was a proprietary solution.

ttf and otf are normal old fonts, so some people got annoyed that this meant anyone could download expensive-to-license fonts for free.

Time passes, SVG 1.1 adds a "fonts" chapter that explains how to model a font purely using SVG markup, and people start to use it. More time passes and it turns out that they are absolutely terrible compared to just a normal font format, and SVG 2 wisely removes the entire chapter again.

Then, woff gets invented by people with quite a bit of domain knowledge, which makes it possible to host fonts in a way that throws away bits that are critically important for system installation, but irrelevant for the web (making people worried about piracy happy) and allows for internal compression to better suit the needs of the web (making users and hosts happy). This becomes the preferred format.

2019 edit A few years later, woff2 gets drafted and accepted, which improves the compression, leading to even smaller files, along with the ability to load a single font "in parts" so that a font that supports 20 scripts can be stored as "chunks" on disk instead, with browsers automatically able to load the font "in parts" as needed, rather than needing to transfer the entire font up front, further improving the typesetting experience.

If you don't want to support IE 8 and lower, and iOS 4 and lower, and android 4.3 or earlier, then you can just use WOFF (and WOFF2, a more highly compressed WOFF, for the newest browsers that support it.)

@font-face {
  font-family: 'MyWebFont';
  src:  url('myfont.woff2') format('woff2'),
        url('myfont.woff') format('woff');
}

Support for woff can be checked at http://caniuse.com/woff
Support for woff2 can be checked at http://caniuse.com/woff2

Woff is a compressed (zipped) form of the TrueType - OpenType font. It is small and can be delivered over the network like a graphic file. Most importantly, this way the font is preserved completely including rendering rule tables that very few people care about because they use only Latin script.

Take a look at [dead URL removed]. The font you see is an experimental web delivered smartfont (woff) that has thousands of combined characters making complex shapes. The underlying text is simple Latin code of romanized Singhala. (Copy and paste to Notepad and see).

Only woff can do this because nobody has this font and yet it is seen anywhere (Mac, Win, Linux and even on smartphones by all browsers except by IE. IE does not have full support for Open Types).

WOFF 2.0, based on the Brotli compression algorithm and other improvements over WOFF 1.0 giving more than 30 % reduction in file size, is supported in Chrome, Opera, and Firefox.

http://en.wikipedia.org/wiki/Web_Open_Font_Format http://en.wikipedia.org/wiki/Brotli

http://sth.name/2014/09/03/Speed-up-webfonts/ has an example on how to use it.

Basically you add a src url to the woff2 file and specify the woff2 format. It is important to have this before the woff-format: the browser will use the first format that it supports.