WOFF 2.0, the inside scoop

Today, WOFF 2.0 is a W3C Recommendation. Perhaps you have some questions: what is it, why should I care, can I use it already?


WebFonts – fonts which are automatically downloaded and used on demand to render a web page, without having to be installed – have risen in importance in recent years. In 2010, a survey of the top 10,000 websites found that only 1% used WebFonts anywhere on the site. That survey is being repeated twice a month, and today, eight years later, 70% use WebFonts.

Work on the original WOFF 1.0 was started in 2010, at which time WebFonts were barely used. WOFF is a container for TrueType and OpenType fonts; the original font data plus a header and optional metadata (such as the name of the designer, a link to a license) are compressed using the same gzip compression used by PNG images. WOFF 1.0 took a minimum viable product approach, using a compression library that was already in the browser. WOFF 1.0 became a W3C Recommendation in December 2012, at which point WebFont usage had risen to 14%.

By the start of 2014, WebFont usage had risen to 35% of websites. With such a volume of data, improving the compression was seen to have significant benefit. A two-stage compression was investigated: firstly a font-specific data reduction, then an improved general compression algorithm.

WOFF 2.0 Font-specific Preprocessing

TrueType and OpenType are designed more for ease of access than for minimal filesize. Data is often padded to an integral number of bytes, making it faster to access. Sometimes the same or similar information is stored at multiple places in the file, for historical reasons. Some data, such as pointers to glyph outlines, is convenient for access but could easily be recalculated. WOFF 2.0 therefore starts by removing redundant or recalculable data, and packing it into the minimum number of bits. This is based on the MicroType Express technology from Monotype.

To take one example,  the bounding box of a single glyph is defined by four numbers (Xmin, Xmax, Ymin, Ymax). The left side bearing is also stored, and is always identical to Xmin. WOFF 2.0 therefore throws away the left side bearing when the font is compressed, and copies over the value from Xmin when the font is reconstructed.

Not all experiments were successful. For example, the placement of knots in a glyph path follows a set of rules, which can (for example, at local curve minima or maxima) allow a point to be predicted based on nearby points and the overall position on the curve. A novel preprocessing stage was considered which removed predictable points before entropy compression, and restored them in the reconstruction step. Experiments showed that a modest reduction in filesize could be obtained, but that the prediction capabilities of the entropy coder were already accounting for these points and thus, the size of the compressed result was not significantly reduced.

WOFF 2.0 compression (entropy coding)

Several options were examined for the second compression stage. One option, LZMA, gave good results but the decoding time was high, which would have been a problem on lower powered devices such as mobile. For this and other reasons, LZMA was abandoned in favor of a novel compression scheme, Brotli. This was first created, and then further developed over the next couple of years, by a Zürich-based expert compression team at Google, and gives excellent compression results; the decompression speed and the memory need to decompress are also very close to what WOFF 1.0 requires.

During development, WOFF2 was tested on three large datasets: the entire Google fonts corpus (1.1k TTF fonts), the Monotype corpus (33k TTF fonts), and the Adobe Fonts corpus, comprising 13.9k TTF fonts and  5k CFF (Type 1)  fonts. This allowed us to not only measure the average compression, but also to examine the best and worst cases. It is worth noting that the WOFF 2.0 size was smaller than the WOFF 1.0 size, for every font tested. Overall, for Truetype fonts (TTF outlines) WOFF 2.0 is 27% better than WOFF 1.0, and for fonts with Type 1 glyphs it is 13.5% better.

In fact, the general-purpose Brotli compressor is so good that it was also adopted into HTTP, providing benefits to the Web for HTML, CSS and Javascript files as well.

WOFF 2.0 and the future

During the development of WOFF 2.0, new font capabilities arose. Multicolored OpenType fonts, and OpenType Variable Fonts are the two main examples. Both are handled by WOFF 2.0 without change, because it is designed to be forwards compatible.

Using WOFF 2.0

WOFF 2.0 is already supported by recent versions of all the major browsers, and has been for the last couple of years. Fallback to WOFF 1.0 for older browsers is easy, a few lines of CSS and no server-side configuration is required:

/* load WOFF 2.0 font if possible, otherwise use WOFF 1.0 font */
@font-face {
  font-family: ExampleName;
  src: url(example.woff2) format("woff2"),
       url(example.woff) format("woff");

Further reading

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