WOFF File Format 2.0

W3C Proposed Recommendation 11 January 2018

This version:
Latest version:
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Latest editor's draft:
Vladimir Levantovsky (Monotype)
Raph Levien (Google)


Based on experience with WOFF 1.0, which is widely deployed, this specification was developed to provide improved compression and thus lower use of network bandwidth, while still allowing fast decompression even on mobile devices. This is achieved by combining a content-aware preprocessing step and improved entropy coding, compared to the Flate compression used in WOFF 1.0.

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.

This document was developed by the WebFonts Working Group as a Candiate Recommendation. This document will remain a Proposed Recommendation at least until in order to ensure the opportunity for wide review. The W3C Membership and other interested parties are invited to review the document and send comments. Advisory Committee Representatives should consult their WBS questionnaires.

A complete list of changes to this document since the last publication is available.

Supporting material, including results of compression measurements, may be found in the companion WOFF 2.0 Evaluation Report. The Working Group has created tests for WOFF2 encoders, decoders, browsers, and for the format itself (a WOFF2 validator). The source to build these tests is available on GitHub.

An Implementation Report is available. Note that, for OpenType Collections (previously, TrueType Collections) while there are two tested implementations of encoding and of decoding, no user agent is yet shown to use them after decoding. Browser support for font collections is expected to improve, and once it does WOFF 2.0 already has the needed support.

Please send comments about this document as GitHub issues.

Publication as a Proposed Recommendation does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

For the Brotli compression scheme used in WOFF 2.0, Google has made an RF licensing commitment for use in this specification.

This document is governed by the 1 March 2017 W3C Process Document.

1. Introduction

This document specifies the WOFF2 font packaging format. This format was designed to provide a reasonably easy-to-implement compression of font data with significantly better compression than previous techniques, suitable for use with CSS @font-face rules. The improvement in compression rates, compared to previously developed WOFF 1.0 format [WOFF1] are realized due to improved entropy coding and font data preprocessing and optimization step that reduces built-in redundancy of various font data structures. The details about WOFF 2.0 development history can be found in the WOFF 2.0 Evaluation Report [WOFF2ER].

1.1 Notational Conventions

The all-uppercase key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. If these words occur in lower- or mixed case, they should be interpreted in accordance with their normal English meaning.

This document includes sections of text that are called out as "Notes" and set off from the main text of the specification. These notes are intended as informative explanations or clarifications, to serve as hints or guides to implementers and users, but are not part of the normative text.

This document also uses hexadecimal numerical notation "0x..." to define the constant binary values such as magic number or tag values, bitfields / flags format and mask values used in pseudo-code bitwise operators.

2. General Requirements

The primary purpose of the WOFF2 format is to efficiently package fonts linked to Web documents by means of CSS @font-face rules. User agents supporting the WOFF2 file format for linked fonts must respect the requirements of the CSS3 Fonts specification ([CSS3-Fonts] Section 4.1: The @font-face rule). In particular, such linked fonts are only available to the documents that reference them; they MUST NOT be made available to other applications or documents on the user's system.

The input font file may contain a number of various font data tables described in the clause 5 of the [OFF] specification. The order of the tables in a font file may differ and, while the entries in the table directory have to be sorted in ascending order by tags, the actual tables can be presented in a font file in arbitrary order.

When WOFF2 file is decompressed, the decoder must sort the entries in the table directory in ascending alphabetical order by table tags and should arrange the table order in a font file according to the Recommendations (subclause 8.1 "Optimized table ordering") of the [OFF].

3. Overall file structure and basic data types

The structure of WOFF2 files is similar to that of SFNT and WOFF 1.0 font files, in that there is a header followed by a table directory, followed by the data for those tables. The SFNT structure is described fully in the TrueType [TrueType], OpenType [OpenType], and ISO "Open Font Format" [OFF] specifications. However, WOFF2 differs in some important aspects from SFNT. Most notably, the data for the font tables is compressed in a single data stream comprising all the font tables. Similar to WOFF 1.0 format, the optional extended metadata and private data blocks are each presented as separate blocks of data, and are concatenated together into the WOFF 2.0 file. The compression algorithm used for both the compressed font data stream and extended metadata block is Brotli [Brotli].

A complete WOFF2 file consists of several blocks of data: a 48-byte header, immediately followed (in this order) by a variable-size Table Directory, an optional Collection Directory (present only if the input font file is a font collection), a compressed font data block, an optional block of Extended Metadata, and an optional block of Private Data. Except for padding with a maximum of three null bytes in places where 4-byte alignment of a table length or block offset is specified, there MUST NOT be any extraneous data between the data blocks or WOFF2 header and table directory, or beyond the last such block or table. If such extraneous data is present a conforming user agent MUST reject the file as invalid. The file MUST also be rejected as invalid if the offsets and lengths of any data blocks or font tables indicate overlapping byte ranges of the file, or ranges that would extend beyond the end of the file.

WOFF2 File
WOFF2Header File header with basic font type and version, along with offsets to metadata and private data blocks.
TableDirectory Directory of font tables, containing size and other info.  
CollectionDirectory An optional table containing the font fragment descriptions of font collection entries.  
CompressedFontData Contents of font tables, compressed for storage in the WOFF2 file.  
ExtendedMetadata An optional block of extended metadata, represented in XML format and compressed for storage in the WOFF2 file.
PrivateData An optional block of private data for the font designer, foundry, or vendor to use.  

3.1. Data types

Data Types
UInt8 8-bit unsigned integer. 
Int16 16-bit signed integer in 2's complement format, stored big-endian. 
UInt16 16-bit unsigned integer, stored big-endian. 
255UInt16 Variable-length encoding of a 16-bit unsigned integer for optimized intermediate font data storage. 
UIntBase128 Variable-length encoding of 32-bit unsigned integers. 

255UInt16 Data Type

255UInt16 is a variable-length encoding of an unsigned integer in the range 0 to 65535 inclusive. This data type is intended to be used as intermediate representation of various font values, which are typically expressed as UInt16 but represent relatively small values. Depending on the encoded value, the length of the data field may be one to three bytes, where the value of the first byte either represents the small value itself or is treated as a code that defines the format of the additional byte(s). The "C-like" pseudo-code describing how to read the 255UInt16 format is presented below:

    Read255UShort( data )
        UInt8 code;
        UInt16 value, value2;

        const oneMoreByteCode1    = 255;
        const oneMoreByteCode2    = 254;
        const wordCode            = 253;
        const lowestUCode         = 253;

        code = data.getNextUInt8();
        if ( code == wordCode ) {
            /* Read two more bytes and concatenate them to form UInt16 value*/
            value = data.getNextUInt8();
            value <<= 8;
            value &= 0xff00;
            value2 = data.getNextUInt8();
            value |= value2 & 0x00ff;
        else if ( code == oneMoreByteCode1 ) {
            value = data.getNextUInt8();
            value = (value + lowestUCode);
        else if ( code == oneMoreByteCode2 ) {
            value = data.getNextUInt8();
            value = (value + lowestUCode*2);
        else {
            value = code;
        return value;

Note that the encoding is not unique. For example, the value 506 can be encoded as [255, 253], [254, 0], and [253, 1, 250]. An encoder may produce any of these, and a decoder MUST accept them all. An encoder should choose shorter encodings, and must be consistent in choice of encoding for the same value, as this will tend to compress better.

UIntBase128 Data Type

UIntBase128 is a different variable length encoding of unsigned integers, suitable for values up to 232-1. A UIntBase128 encoded number is a sequence of bytes for which the most significant bit is set for all but the last byte, and clear for the last byte. The number itself is base 128 encoded in the lower 7 bits of each byte. Thus, a decoding procedure for a UIntBase128 is: start with value = 0. Consume a byte, setting value = old value times 128 + (byte bitwise-and 127). Repeat last step until the most significant bit of byte is false.

UIntBase128 encoding format allows a possibility of sub-optimal encoding, where e.g. the same numerical value can be represented with variable number of bytes (utilizing leading 'zeros'). For example, the value 63 could be encoded as either one byte 0x3F or two (or more) bytes: [0x80, 0x3f]. An encoder must not allow this to happen and must produce shortest possible encoding. A decoder MUST reject the font file if it encounters a UintBase128-encoded value with leading zeros (a value that starts with the byte 0x80), if UintBase128-encoded sequence is longer than 5 bytes, or if a UintBase128-encoded value exceeds 232-1. The "C-like" pseudo-code describing how to read the UIntBase128 format is presented below:

    bool ReadUIntBase128( data, *result )
        UInt32 accum = 0;

        for (i = 0; i < 5; i++) {
            UInt8 data_byte = data.getNextUInt8();

            // No leading 0's
            if (i == 0 && data_byte = 0x80) return false;

            // If any of top 7 bits are set then << 7 would overflow
            if (accum & 0xFE000000) return false;

            *accum = (accum << 7) | (data_byte & 0x7F);

            // Spin until most significant bit of data byte is false
            if ((data_byte & 0x80) == 0) {
                *result = accum;
                return true;
        // UIntBase128 sequence exceeds 5 bytes
        return false;

3.2. WOFF2 Header

The WOFF 2.0 header includes an identifying signature and provides the information about the compressed and uncompressed sizes of encapsulated font data. It also indicates the specific kind of font data included in the WOFF 2.0 file, font version number and provides offsets to additional data blocks included in the file.

WOFF2 Header
UInt32signature0x774F4632 'wOF2'
UInt32flavorThe "sfnt version" of the input font.
UInt32lengthTotal size of the WOFF file.
UInt16numTablesNumber of entries in directory of font tables.
UInt16reservedReserved; set to 0.
UInt32totalSfntSizeTotal size needed for the uncompressed font data, including the sfnt header,
directory, and font tables (including padding).
UInt32totalCompressedSizeTotal length of the compressed data block.
UInt16majorVersionMajor version of the WOFF file.
UInt16minorVersionMinor version of the WOFF file.
UInt32metaOffsetOffset to metadata block, from beginning of WOFF file.
UInt32metaLengthLength of compressed metadata block.
UInt32metaOrigLengthUncompressed size of metadata block.
UInt32privOffsetOffset to private data block, from beginning of WOFF file.
UInt32privLengthLength of private data block.

The interpretation of the WOFF2 Header is the same as the WOFF Header in [WOFF1], with the addition of one new totalCompressedSize field. The signature field in the WOFF2 header MUST contain the value of 0x774F4632 ('wOF2'), which distinguishes it from WOFF 1.0 files. If the field does not contain this value, user agents MUST reject the file as invalid. A valid WOFF 2.0 file MUST have the reserved field set to 'zero', a decoder MUST NOT reject a downloaded font file if the reserved header value is not zero. User agents MUST NOT reject correctly decoded font file if the resulting font file size doesn't match the totalSfntSize value encoded as part of the WOFF2 header.

The "totalSfntSize" value in the WOFF2 Header is intended to be used for reference purposes only. It may represent the size of the uncompressed input font file, but if the transformed 'glyf' and 'loca' tables are present, the uncompressed size of the reconstructed tables and the total decompressed font size may differ substantially from the original total size specified in the WOFF2 Header.

4. Font directory

The font directory section consists of the table directory, containing a single entry for each unique font table in the input font file, and may also include an optional collection directory table, when the input font file is a font collection.

4.1. Table directory format

The table directory is an array of WOFF2 table directory entries, as defined below. The directory follows immediately after the WOFF2 file header; therefore, there is no explicit offset in the header pointing to this block. Its size is dependent on the exact content; thus, the best strategy for decoding is to process the file as a stream, rather than trying to access it randomly. Each table directory entry specifies the original size of a single font data table and, for those tables where an additional transform has been applied, the size of the transformed table. Transformed tables are identified by their 'flags' field that combines known table tags (bits 0-5, see "Known Table Tags") and transformation version number (bits 6 and 7, see the description below) defining the applied transform. The combination of the table tag and the transform version number defines the exact transform that was applied to a table, currently defined transforms are specified in details in clause 5 of this specification.

Contrary to the way how table directory entries are specified in the original input font file (where table directory entries are sorted in ascending alphabetical order), the WOFF2 table directory entries define the physical order of tables in which they have been processed and encoded as part of the compressed font data stream. It is a decoder responsibility to sort and reorder the reconstructed table directory when the font file is decompressed.

In order to simplify the font decoding process an encoder MUST reorder the transformed tables so that the associated 'glyf' and 'loca' tables are always present as a pair, with 'loca' table following the 'glyf' table. Please keep in mind that the entry for transformed 'loca' table is just a placeholder indicating the size required to reconstruct the table while consuming no bytes of the data stream. Therefore, the requirement for the 'loca' table entry to immediately follow the transformed 'glyf' table in the table directory makes it easier to implement the 'loca' table reconstruction step when the reverse transform (see subclause 5.1 for details) is being applied to the 'glyf' table. For more details on table order constraints see subclause 5.5.

The format of each individual table directory entry is as follows:

UInt8flagstable type and flags
UInt32tag4-byte tag (optional)
UIntBase128origLengthlength of original table
UIntBase128transformLengthtransformed length (if applicable)

The interpretation of the flags field is as follows. Bits [0..5] contain an index to the "known tag" table, which represents tags likely to appear in fonts. If the tag is not present in this table, then the value of this bit field is 63. Bits 6 and 7 indicate the preprocessing transformation version number (0-3) that was applied to each table. For all tables in a font, except for 'glyf' and 'loca' tables, transformation version 0 indicates the null transform where the original table data is passed directly to the Brotli compressor for inclusion in the compressed data stream. For 'glyf' and 'loca' tables, transformation version 3 indicates the null transform where the original table data was passed directly to the Brotli compressor without applying any pre-processing defined in subclause 5.1 and subclause 5.3. The transformed table formats and their associated transformation version numbers are described in details in clause 5 of this specification.

If multiple transformation versions are defined for a given table, an encoder should normally choose the transformation version that results in the smallest transformed table length, unless the reduction is so slight as to be insignificant.

Where more than one transformation version is defined for a given table tag, an encoder may use any of the defined transformations that is applicable to the input data; a decoder MUST be prepared to accept any of the defined transformations. If a decoder encounters a table entry that specifies an unknown transformation version number the entire font MUST be rejected as it cannot be correctly decoded.

Whether a font table is encoded with a known table tag or explicitly including the four-byte tag has no semantical significance; it is simply a choice of encoding intended to improve compression efficiency. Similarly, whether a particular four-byte tag is present or not in the "Known Table Tags" table is not a normative statement about whether such tags should be included in web fonts.The encoder MUST use known table flag when encoding an input font table with a tag that is listed in the "Known Table Tags" table.

There is a predefined extension mechanism for any custom table tag that is not included in the known tag list (or for any new standard tags that may be defined in the future). If bits [0..5] of the flags byte have the value 63 (0x3f), then following the flag byte is a 4-byte arbitrary tag value. Otherwise, the tag field is omitted in the TableDirectoryEntry structure, and is derived from bits [0..5] of the flag byte from the fixed Known Table Tags table, given below. The decoder MAY accept a table entry defined using a custom table tag even if the tag that follows is identified as one of the kown table tags.

Known Table Tags
Flag Tag Flag Tag Flag Tag Flag Tag
4maxp20LTSH36SVG 52mort
8cvt 24vmtx40bdat56trak
13CFF 29EBSC45feat61Feat
15EBDT31MATH47fvar63arbitrary tag follows

Please note that according to the SFNT-based font format specifications all table tags should consist of four characters. Table tags with less than four letters, such as 'cvt ' (tag value 0x63767420) are padded with trailing spaces.

Following the flags (and the optional tag field) are one or two length values, each in UIntBase128 unsigned integer encoding. The origLength field specifies the length of the table in an uncompressed version of the font. For tables that are subjected to additional transformations, the transformLength field that specifies the length of the transformed version of the table MUST be included.

The transformLength field is present in the table directory entry if, and only if, the table has been processed by a non-null transform prior to Brotli compression. For tables that are not transformed, no transformLength field is present in the directory entry.

The field containing the transformed length provides information about the transformed table size prior to it being compressed by [Brotli]. This field is optional and is present only for certain tables (see below the description of the "Compressed data format"). Please note that while the transformed length can be relied upon to determined the decompressed table size, the original table length of the transformed font table should be treated with caution.

The reconstruction process of transformed tables guarantees preserving the functionality of the tables but may produce binary results that are different from the original data. For example, 'glyf' table records may have outline point coordinates encoded using one- or two-byte format, and the repetitive values can either be explicitly duplicated in the coordinate stream or omitted (which would be indicated by the flags byte of the control point). Therefore, various representations of glyph outline points are possible that would produce identical rendering results; however, the binary data of reconstructed glyph records may differ significantly from the original data. See subclause 5.3.3 of the ISO "Open Font Format" [OFF] specification for details.

For this reason, the value of the origLength field of transformed table should be treated only as a reference and should not be relied upon in making memory allocation decisions when the WOFF2 data is decoded.

4.2. Collection directory format

The collection directory is present only if the input font is a collection; i.e. if the value of the WOFF2 file header "flavor" field is set to 0x74746366 ('ttcf'). If present, the collection directory follows immediately after the table directory. The collection directory consists of a CollectionHeader and one or more CollectionFontEntry records.

If input font is a collection, the table directory contains a single entry for each unique table in the entire collection, regardless of whether a table is shared among different fonts in a collection or not. Thus, the table directory for a font collection compressed with WOFF2 comprises all tables from all fonts contained in the font collection file.

The Collection directory starts with the Collection Header followed by an array of Collection Font Entries.

UInt32versionThe Version of the TTC Header in the original font.
255UInt16numFontsThe number of fonts in the collection.

Collection font entries specify each font in the collection (defined by CollectionHeader "numFonts" field). The format of each individual collection font entry is as follows:

255UInt16numTablesThe number of tables in this font
UInt32flavorThe "sfnt version" of the font
255UInt16index[numTables]The index identifying an entry in the Table Directory for each table in this font (where the index of the first Table Directory entry is zero.)

A table whose offset is referred to by multiple Offset Tables in the original collection is considered "shared". An encoder MUST emit a single TableDirectoryEntry for each unique font table defined by its offset, regardless of whether that table is shared among multiple fonts in a collection or not. I.e., if a collection contains multiple fonts with their corresponding Table Records referencing shared font tables with the same offsets, the table data in the WOFF2 compressed data stream MUST NOT be duplicated.

If an input font file is a font collection that contains multiple unique glyf and loca tables, the transformations described for the glyf and loca tables MUST be applied to each pair of the tables.Sharing of glyf and loca tables is allowed and encouraged (this is one of the major benefits of a font collection); however, it is possible that font collections may have two or more pairs of the glyf / loca tables that may not be shared. When the tables are shared, an encoder MUST verify that both tables are shared and that both form an associated pair (if more than one pair of glyf / loca tables are present) and MUST reject a collection containing fonts that share only one of either glyf or loca table.

An encoder, when processing a font collection, MUST preserve the same order of the nested fonts as they are in the input font collection and MUST record the index of the matching TableDirectoryEntry into the CollectionFontEntry for each font.

A decoder MUST restore the collection with the same number of fonts and their corresponding Offset Tables. The Offset Tables for each font that comprises the output font collection file MUST have either the same numTables as the input collection, or one less if DSIG was present in the original input Offset Table. If the value of the version field for the TTC Header in the CollectionHeader is set to "2.0", a decoder MUST either set the TTC Header fields {ulDsigTag, ulDsigLength, ulDsigOffset} in the output collection to null or convert the TTC header format to version 1 (0x00010000).

A decoder, when processing a font collection, MUST recreate the same order of the nested fonts as they were in the input collection. The tables within each nested font can be reordered. When processing a font collection with multiple pairs of 'glyf'/'loca' tables present, a decoder MUST check CollectionFontEntry indices to make sure that referenced 'glyf' and 'loca' tables are paired with each other, and MUST reject a font collection if a mismatch in table pairing is found. For more details about table pairs see the table directory format description. A conforming user agent MUST be able to load and use properly formatted font collection file generated by a WOFF2 decoder.

5. Compressed data format

The CompressedFontData field in a WOFF2 file contains the concatenation of data for each table in the font, in the order that entries appear in the WOFF2 table directory. There MUST NOT be any extraneous data between the table entries in the decompressed data stream as defined by the table directory. If such extraneous data is present a conforming user agent MUST reject the file as invalid. The CompressedFontData stream MUST be compressed using the Brotli compression algorithm [Brotli]. If the decompression of the data block fails for any reason, the WOFF2 file is invalid and MUST NOT be loaded.

The process of decoding the table data in a WOFF2 font file can be specified by decompressing the byte-level compression of the CompressedFontData field, yielding a "table data block", then applying additional decoding steps as described below. An actual implementation is free to combine these steps or perform some of the steps in an incremental or streaming fashion, but the results must be consistent with the sequential process as specified here.

Certain tables (such as glyf, loca and hmtx tables, identified by their corresponding tags), are subject to additional transforms. If a font table is not transformed, then the table data appears in the compressed stream in literal form, and occupies origLength bytes of the table data block. Whether or not the table is transformed, and the version number of the applied transform is defined by the two most significant flag bits (see subclause 4.1 for details), and the table data must be additionally processed by a transformation specified below. In this case, the transformed table occupies transformLength bytes of the table data block.

The decompressed and reconstructed table data MUST be stored in the format specified by the [OFF] specification. Each reconstructed table directory entry MUST contain a valid 'checkSum' value, the decoder MUST recalculate the checkSum value for each decoded table. Also, due to modifying transforms applied to glyf and loca tables, the decoder MUST recalculate the checkSumAdjustment value of the entire font and MUST store the updated value in the head table.

The known table flag values should not be relied upon in determining the presence of transformed tables, it is feasible that e.g. the glyf table can be represented in the table directory with either flag = 10 and no tag, or with flag = 63 and 'glyf' tag that follows. The combination of flags value that defines a transform version number and the table tag should be used to determine the applied transform.

The sum of the origLength (for non-transformed tables) and transformLength (for transformed tables) fields in the table directory MUST equal the size of the font data block after it has been decompressed using [Brotli].If the size of the decompressed font data block doesn't match the sum of lengths defined in the table directory as described above, the WOFF2 file MUST NOT be loaded.

According to the curent version of the WOFF2, a transform can be applied to three types of tables: glyf, representing outline data, loca, representing the offsets of the individual glyphs within a glyf table, and hmtx, representing glyph horizontal metrics, if these tables are present in a font. An authoring tool can use any applicable transforms taking into consideration additional constraints specified in subclause 5.3 and subclause 5.5. The glyf table transformation is specified in subclause 5.1, the loca table transformation is specified in subclause 5.3, and the hmtx table transformation is specified in subclause 5.4.

CFF table processing and de-subroutinization.

The majority of OpenType fonts with CFF outlines have CFF data subroutinized for more compact data storage. The experiments conducted as part of WOFF2 development and evaluation have shown that Brotli compression technology works more efficiently and produces better compression gains for WOFF2 fonts if the CFF outline data is de-subroutinized prior to WOFF2 encoding - this will on average reduce the compressed data size by 5-10%. However, the CFF de-subroutinization will, at the same time, increase the output file size by ~10%.

Since the CFF pre-processing / de-subroutinization is considered an external step that can be implemented prior to WOFF2 encoding, it is not covered by this specification. Font producers have an opportunity to optimize their production process to either reduce the compressed font size for fastest webfont data transfer at the expense of increasing the output CFF font size (which will also improve rendering performance) or to keep subroutinized CFF data to minimize input/output font size.

The WOFF 2.0 transformations applied to certain tables are designed to reduce and/or eliminate the built-in redundancies of the SFNT format and restructure the font data stream for more efficient entropy encoding. As a result, the reconstructed font data will retain the exact functionality of the input font file, but due to certain possible encoding variations (such as various levels of optimization of outline point coordinates in the 'glyf' table, and/or difference in offset calculations of the 'loca' table) different WOFF2 decoders may produce an output file that will not be a bitwise match to the input font file. The font tables of the input font file may also be subjected to reordering operations, which may take place either as a part of the encoding process or the tables can be reordered by a decoder to comply with the recommended table order defined by the [OFF] specification – as a result, even if the content of the font data tables is not affected, the table offsets may change. The differences in binary font data (however minor, due to significant extent to decoder optimizations and table reordering) between encoded and decoded font data structures will invalidate the 'DSIG' table, if one is present. Therefore, the compliant WOFF2 encoder MUST remove the DSIG table from an input font data, prior to applying transformations and entropy coding steps.

The WOFF 2.0 encoders MUST also set bit 11 of the 'flags' field of the head table (see [OFF] specification) to indicate that a recreated font file was subjected to lossless modifying transform.

5.1. Transformed glyf table format

The glyf table data can be presented in the WOFF2 file in one of two formats defined by the transformation version number (encoded in the table directory flag bits, see subclause 4.1 for details). The transformation version "3" defines a null transform where the content of the glyf table is presented in its original, unmodified format. The transformation version "0", specified below, is optional and can be applied to eliminate certain redundancies in the glyf table data.

The version "0" of the glyf table transformation (as defined by the table directory flag bits, see subclause 4.1 for details) specified in this subclause is intended to reduce redundant information and provide a more efficient encoding of the actual TrueType outlines of glyphs. The modified transformation is based on a similar transformation described in MicroType Express [MTX] specification. The reference to MTX is informative; the details of the modified transformation are stated below and this section is normative.

While glyf table transformation preserves the functionality and fidelity of rendering for every glyph in a font, it will most likely result in producing a new set of glyph records that will not be a binary match to the original font. According to the encoding rules specified in subclause 5.3.3 of [OFF], there may be multiple valid reconstructions of a glyph record. It is possible that depending on the context and on the level of optimizations being applied to reconstructed glyf table data, the size of the reconstructed table may be greater than, less than, or equal to the original size of the glyf table of the input font compressed by WOFF2. Neither this specification nor the [OFF] standard mandate specific optimizations being applied on the font data; therefore, it is the responsibility of WOFF2 decoder implementations to make sure that an adequate amount of memory is allocated when glyph records are being reconstructed from the WOFF2 file – the original glyf table size supplied as an origLength value for the transformed glyf table can only be used as a reference point.

Regardless of the implementation details of the glyph record reconstruction process and the level of optimizations applied to reconstructed 'glyf' table data User Agents MUST NOT reject correctly decoded font file if the size of the reconstructed 'glyf' table doesn't match the origLength value encoded as part of the 'glyf' table entry in WOFF2 Table Directory.

For greater compression effectiveness, the glyf table is split into several substreams, to group like data together. The transformed table consists of a number of fields specifying the size of each of the substreams, followed by the substreams in sequence. During the decoding process the reverse transformation takes place, where data from various separate substreams are recombined to create a complete glyph record for each entry of the original glyf table.

Transformed glyf Table
Data Type Semantic Description and value type (if applicable)
Fixedversion= 0x00000000
UInt16numGlyphsNumber of glyphs
UInt16indexFormatOffset format for loca table, should be consistent with indexToLocFormat of the original head table (see [OFF] specification)
UInt32nContourStreamSizeSize of nContour stream in bytes
UInt32nPointsStreamSizeSize of nPoints stream in bytes
UInt32flagStreamSizeSize of flag stream in bytes
UInt32glyphStreamSizeSize of glyph stream in bytes (a stream of variable-length encoded values, see description below)
UInt32compositeStreamSizeSize of composite stream in bytes (a stream of variable-length encoded values, see description below)
UInt32bboxStreamSizeSize of bbox data in bytes representing combined length of bboxBitmap (a packed bit array) and bboxStream (a stream of Int16 values)
UInt32instructionStreamSizeSize of instruction stream (a stream of UInt8 values)
Int16nContourStream[]Stream of Int16 values representing number of contours for each glyph record
255UInt16nPointsStream[]Stream of values representing number of outline points for each contour in glyph records
UInt8flagStream[]Stream of UInt8 values representing flag values for each outline point.
VaryglyphStream[]Stream of bytes representing point coordinate values using variable length encoding format (defined in subclause 5.2)
VarycompositeStream[]Stream of bytes representing component flag values and associated composite glyph data
UInt8bboxBitmap[]Bitmap (a numGlyphs-long bit array) indicating explicit bounding boxes
Int16bboxStream[]Stream of Int16 values representing glyph bounding box data
UInt8instructionStream[]Stream of UInt8 values representing a set of instructions for each corresponding glyph

The format is best characterized by describing the decoding process, especially indications of what are valid and invalid data. Note also that this format specifies the decoded result at the semantic level, not specific byte streams.

Included in the Transformed glyf Table is a bboxBitmap indicating for each glyph whether it contains an explicitly encoded bounding box, or whether the bounding box is to be inferred from the coordinate values. The relevant computations to determine the bounding box status must be performed by both an encoder and a decoder. For each simple glyph, an encoder MUST calculate the the xMin, yMin, xMax and yMax coordinate values using all outline points (both on- and off-curve points) and compare the calculated values with the encoded bounding box info for the glyph. If the glyph bounding box info matches the calculated values, an encoder MUST omit the bounding box info. Otherwise, if the calculated bounding box values do not equal the glyph values encoded in a font, an encoder MUST set the corresponding bboxBitmap flag and record the original bounding box values in the bboxStream. For glyphs records that contain zero contours, an encoder MUST check that the glyph bounding box values are all zeros and, if this condition is not met, MUST reject an input font as invalid. For either an empty glyph or a glyph with zero contours, an encoder MUST always clear the corresponding bboxBitmap flag. For a composite glyph, an encoder MUST always set the corresponding bboxBitmap flag and record the original bounding box values in the bboxStream.

A decoder MUST perform similar calculations if the bounding box is to be inferred. For a given glyph, when a corresponding bit in bboxBitmap is not set, the xMin, yMin, xMax and yMax values MUST be calculated at the time of decoding using all outline point coordinates. The total number of bytes in bboxBitmap is equal to 4 * floor((numGlyphs + 31) / 32). The bits are packed so that glyph number 0 corresponds to the most significant bit of the first byte, glyph number 7 corresponds to the least significant bit of the first byte, glyph number 8 corresponds to the most significant bit of the second byte, and so on. A bit=1 value indicates an explicitly set bounding box.

Upon reading the Transformed glyf Table, the decoding process iterates one glyph at a time. For each glyph, it reads zero or more bytes from each of the streams referenced in the Transformed glyf Table. Also, at the point of reconstructing a glyph, a decoder needs to store for each glyph the corresponding offset in the new reconstructed glyph table, and this data will collectively become the contents of the reconstructed loca table (see subclause 5.3 below for more information about the reconstruction of the loca table).

Reconstruction of Glyph Records

The reconstruction process begins by performing the following step to determine the encoded glyph type (simple, composite or empty glyph):

Read a Int16 from the nContour stream. Store this in the numberOfContours field in the reconstructed TrueType glyph. The interpretation of the field is the same as the TrueType spec; if it is zero, the glyph is empty. If it is positive, the glyph is simple and the value represents the number of contours in the outline. If the nContour value is equal to -1 (0xffff), then the glyph is composite.

Decoding of Empty Glyphs

Reconstruction of an empty glyph (when nContour = 0) is a simple step that involves incrementing the glyph record count and creating a new entry in the loca table where loca[n] = loca[n-1]. If the bboxBitmap flag indicates that the bounding box values are explicitly encoded in the bboxStream the decoder MUST reject WOFF2 file as invalid.

Decoding of Simple Glyphs

For a simple glyph (when nContour > 0), the process continues as follows:

  1. Read numberOfContours 255UInt16 values from the nPoints stream. Each of these is the number of points of that contour. Convert this into the endPtsOfContours[] array by computing the cumulative sum, then subtracting one. For example, if the values in the stream are [2, 4], then the endPtsOfContours array is [1, 5]. Also, the sum of all the values in the array is the total number of points in the glyph, nPoints. In the example given, the value of nPoints is 6.
  2. Read nPoints UInt8 values from the flags stream. Each corresponds to one point in the reconstructed glyph outline. The interpretation of the flag byte is described in details in subclause 5.2.
  3. For each point (i.e. nPoints times), read a number of point coordinate bytes from the glyph stream. The number of point coordinate bytes is a function of the flag byte read in the previous step: for (flag < 0x7f) in the range 0 to 83 inclusive, it is one byte. In the range 84 to 119 inclusive, it is two bytes. In the range 120 to 123 inclusive, it is three bytes, and in the range 124 to 127 inclusive, it is four bytes. Decode these bytes according to the procedure specified in the subclause 5.2 to reconstruct delta-x and delta-y values of the glyph point coordinates. Store these delta-x and delta-y values in the reconstructed glyph using the standard TrueType glyph encoding [OFF] subclause 5.3.3.
  4. Read one 255UInt16 value from the glyph stream, which is instructionLength, the number of instruction bytes.
  5. Read instructionLength bytes from instructionStream, and store these in the reconstituted glyph as instructions.

Decoding of Composite Glyphs

For a composite glyph (nContour == -1), the following steps take the place of steps 1-5 above:

1a. Read a UInt16 from compositeStream. This is interpreted as a component flag word as in the TrueType spec. Based on the flag values, there are between 4 and 14 additional argument bytes, interpreted as glyph index, arg1, arg2, and optional scale or affine matrix.

2a. Read the number of argument bytes as determined in step 2a from the composite stream, and store these in the reconstructed glyph. If the flag word read in step 2a has the FLAG_MORE_COMPONENTS bit (bit 5) set, go back to step 2a.

3a. If any of the flag words had the FLAG_WE_HAVE_INSTRUCTIONS bit (bit 8) set, then read the instructions from the glyph and store them in the reconstructed glyph, using the same process as described in steps 4 and 5 above.

Finally, for both simple and composite glyphs, if the corresponding bit in the bounding box bit vector is set, then additionally read 4 Int16 values from the bbox stream, representing xMin, yMin, xMax, and yMax, respectively, and record these into the corresponding fields of the reconstructed glyph. For simple glyphs, if the corresponding bit in the bounding box bit vector is not set, then derive the bounding box by computing the minimum and maximum x and y coordinates in the outline, and storing that.

A composite glyph MUST have an explicitly supplied bounding box. The motivation is that computing bounding boxes is more complicated, and would require resolving references to component glyphs taking into account composite glyph instructions and the specified scales of individual components, which would conflict with a purely streaming implementation of font decoding. A decoder MUST check for presence of the bounding box info as part of the composite glyph record and MUST NOT load a font file with the composite bounding box data missing.

5.2. Decoding of variable-length X and Y coordinates

Simple glyph data structure defines all contours that comprise a glyph outline, which are presented by a sequence of on- and off-curve coordinate points. These point coordinates are encoded as delta values representing the incremental values between the previous and current corresponding X and Y coordinates of a point, the first point of each outline is relative to (0,0) point. To minimize the size of the dataset of point coordinate values, each point is presented as a (flag, xCoordinate, yCoordinate) triplet. The flag value is stored in a separate data stream and the coordinate values are stored as part of the glyph data stream using a variable-length encoding format consuming a total of 2-5 bytes per point.

The most significant bit of a flag indicates whether the point is on- or off-curve point, the remaining seven bits of the flag determine the format of X and Y coordinate values and specify 128 possible combinations of indices that have been assigned taking into consideration typical statistical distribution of data found in TrueType fonts. When X and Y coordinate values are recorded using nibbles (either 4 bits per coordinate or 12 bits per coordinate) the bits are packed in the byte stream with most significant bit of X coordinate first, followed by the value for Y coordinate (most significant bit first). As a result, the size of the glyph dataset is significantly reduced, and the grouping of the similar values (flags, coordinates) in separate and contiguous data streams allows more efficient application of the entropy coding applied as the second stage of encoding process.

Each of the 128 index values define the following properties and specified in details in the table below:

Please note that “Byte Count” field reflects total size of the triplet (flag, xCoordinate, yCoordinate), including ‘flag’ value that is encoded in a separate stream.

Triplet Encoding
Index Byte Count X bits Y bits Delta X Delta Y X sign Y sign

For additional information and background on the triplet encoding please see section 5.11 of the MTX proposal [MTX].

5.3. Transformed loca table format

The loca table data can be presented in the WOFF2 file in one of two formats defined by the transformation version number (encoded in the table directory flag bits, see subclause 4.1 for details). The transformation version "3" defines a null transform where the content of the loca table is presented in its original, unmodified format. The transformation version "0", although optional, MUST be applied to the loca table data whenever glyf table data is transformed. In other words, both glyf and loca tables must either be present in their transformed format or with null transform applied to both tables.

The version "0" of the loca table transformation (as defined by the table directory flag bits, see subclause 4.1 for details) is specified below.

The transformLength of the transformed loca table MUST always be zero. The origLength MUST be the appropriate size (determined by numGlyphs+1, times the size per glyph, where that size per glyph is two bytes when indexFormat (defined in subclause 5.1. Transformed glyf table format) is zero, otherwise four bytes). If the transformLength of the transformed loca table is not equal to zero, or if the encoded origLength does not match the calculated size defined above the decoder MUST reject the WOFF2 file as invalid.

The loca table MUST be reconstructed when the glyf table is decoded. The process for reconstructing the loca table is specified in subclause 5.1 as part of the transformed glyf table decoding process. For reconstructed glyph records, a decoder MUST store the corresponding offsets for individual glyphs using a format that is indicated by the indexFormat field of the Transformed glyf Table.

5.4. Transformed hmtx table format

The hmtx table data can be presented in the WOFF2 file in one of two formats defined by the transformation version number (encoded in the table directory flag bits, see subclause 4.1 for details). The transformation version "0" defines a null transform where the content of the hmtx table is presented in its original, unmodified format. The transformation version "1", specified below, is optional and can be applied to eliminate certain redundancies in the hmtx table data.

The transformation version "1" exploits the built-in redundancy of the TrueType glyphs where the outlines of the glyphs designed according to the TrueType recommendations would likely have their left side bearing values equal to xMin value of the glyph bounding box. The hmtx table format is specified in subclause 5.2.4 of [OFF] and includes two arrays of left side bearing information for proportional and monospaced glyph runs. Each of these arrays can be eliminated from the original input font data and reconstructed from the glyph bounding box information if certain conditions are met.

Transformed hmtx table format
Data Type Semantic Description and value type (if applicable)
UInt8 Flags Bitfield indicating the specific details of the applied transform:
Bit 0 - when set, indicates that lsb[] array is not present and needs to be reconstructed from the xMin values in the glyf table;
Bit 1 - when set, indicates that leftSideBearing[] array is not present and needs to be reconstructed from the xMin values in the glyf table.
When hmtx transform is indicated by the table directory, the Flags (bits 0 or 1 or both) MUST be set. Bits 2-7 are reserved and MUST be zero.
UInt16 advanceWidth[] Stream of Uint16 values representing the advanceWidth values of the horizontal metrics array for proportional glyphs. The stream size is equal to numOfHMetrics defined in the hhea table.
Int16 lsb[] Stream of Int16 values representing the left side bearing values of the horizontal metrics array for proportional glyphs. The stream is present only if the bit 0 of the "Flags" field is not set, otherwise this stream is missing. The stream size is equal to numOfHMetrics defined by the hhea table.
Int16 leftSideBearing[] Stream of Int16 values representing the left side bearing values of the horizontal metrics array for monospaced glyphs. The stream is present only if the bit 1 of the "Flags" field is not set, otherwise this stream is missing. The stream size is equal to (numGlyphs - numOfHMetrics).

The transformation version 1 described in this subclause is optional and can only be used when an input font is TrueType-flavoured (i.e. has a glyf table), and when the leftSideBearing values for each glyph in the proportional or monospaced glyph runs encoded in the hmtx table exactly match the corresponding xMin values in the glyf table. A special case needs to be checked when an encoded leftSideBearing value corresponds to an empty glyph, where hmtx table transform can only be applied if leftSideBearing of an empty glyph is set to zero.

If the hmtx table transform is both applicable and desired, the encoder MUST check that leftSideBearing values match the xMin values of the glyph bounding box for every glyph in a font (or check that leftSideBearing == 0 for an empty glyph) and, for font collections, this check MUST be performed on every corresponding pair of glyf and hmtx tables. If the conditions are met for each of the proportional or monospaced glyph runs the encoder MUST set hmtx transform version number to "1", MUST eliminate the corresponding array from the hmtx table and MUST set the appropriate Flags bits. In font collections that have shared hmtx table, the match between xMin values of glyph bounding boxes and the lsftSideBearing values MUST be checked for all combinations of shared hmtx and various glyf tables, and if there is a mismatch the hmtx table transform MUST NOT be applied.

When the table directory flags indicate that the hmtx transform is applied, the User Agent MUST check the Flags values and MUST reject the WOFF2 file if Flags are invalid. The decoder MUST reconstruct the lsb[] and/or leftSideBearing[] arrays from the xMin values provided by the glyph bounding box information from the glyf table.

5.5. Table order constraints

The following constraints on valid WOFF2 files are intended to facilitate a memory-efficient WOFF 2.0 file transfer and decoding process.

The loca table MUST follow the glyf table in the table directory. When WOFF2 file contains individually encoded font file, the table directory MAY contain other tables inserted between glyf and loca tables; however when WOFF2 contains a font collection file each loca table MUST immediately follow its corresponding glyf table. For example, the following order of tables: 'cmap', 'glyf', 'hhea', 'hmtx', 'loca', 'maxp' ... is acceptable for individually encoded font files; however, font collection files must have glyf / loca tables paired with each other, although they don't have to be ordered as a contiguous block of tables, e.g.: 'cmap', 'glyf', 'loca', 'hhea', 'hmtx', 'glyf', 'loca', 'maxp', 'post' ... would be acceptable. Additional constraints on the loca table are given in subclause 5.3.

6. Extended Metadata Block

The WOFF2 file MAY contain a block of extended metadata. The interpretation of this block is exactly the same as [WOFF 1].

However, while in a WOFF 1 file the extended metadata block is stored with zlib compression, in a WOFF2 file the metadata MUST be stored compressed with [Brotli] as a separate stream, if present.. The rationale of this change is to minimize the total number of byte-level compression algorithms needed to implement WOFF2, and also because Brotli is expected to achieve better compression ratios than zlib in most cases.

The metadata block MUST follow immediately after the compressed font data stream. The beginning of the metadata block MUST always be 4-byte aligned; when extended metadata is present, the end of the compressed data block must be padded with up to three null bytes if needed to reach a 4-byte boundary. If the metadata block is the last block in the WOFF file, there SHOULD be no additional padding after the end of the block.

7. Private Data Block

The WOFF2 file MAY include a block of arbitrary data, allowing font creators to include whatever information they wish. The content of this data MUST NOT affect font usage or load behavior of user agents. User agents should make no assumptions about the content of a private block; it may (for example) contain ASCII or Unicode text, or some vendor-defined binary data, and it may be compressed or encrypted, but it has no publicly defined format. Conformant user agents will not assume anything about the structure of this data. Only the font developer or vendor responsible for the private block is expected to understand its contents.

The private data block, if present, MUST be the last block in the WOFF file, following the compressed font data stream and the extended metadata block, if one is present. The private data block MUST begin on a 4-byte boundary in the WOFF file, with up to three null bytes inserted as padding after any preceding block, if needed to ensure this. The end of the private data block MUST correspond to the end of the WOFF2 file.

A. Internet Media Type Registration

This appendix registers a new Internet Media type, under the font top level type [RFC8081]. This appendix is in conformance with BCP 13 and W3CRegMedia.

WOFF 2.0
Type name:


Subtype name:


Required parameters:


Optional parameters:


Encoding considerations:


Interoperability considerations:

WOFF 2.0 is an improvement on WOFF 1.0. The two formats have different Internet Media Types and may be used in parallel.

Published specification:

This media type registration is extracted from the WOFF 2.0 specification at W3C.

Applications that use this media type:

WOFF 2.0 is used by Web browsers, often in conjunction with HTML and CSS.

Additional information:
Magic number(s):
The signature field in the WOFF header MUST contain the "magic number" 0x774F4632 ('wOF2')
File extension(s):
Macintosh file type code(s):
(no code specified)
Macintosh Universal Type Identifier code:
@font-face Format:
Fragment Identifiers
See Section 4.2 of [RFC8081]
Person & email address to contact for further information:

Chris Lilley (www-font@w3.org).

Intended usage:


Restrictions on usage:



The WOFF2 specification is a work product of the World Wide Web Consortium's WebFonts Working Group.

Change controller:

The W3C has change control over this specification.

B. Changes

This appendix is informative.

Changes since the Candidate Recommendation of 15 March 2016

Changes since the Working Draft of 14 April 2015

A color-coded diff between the 14 April 2015 Working Draft and the Editors's Draft used to prepare the Candidate Recommendation is available.

Changes since the First Public Working Draft of May 8, 2014

A color-coded diff between the editors draft used to prepare the First Public Working Draft and the editors draft used to prepare the Last Call Working Draft is available.

C. Considerations for Security and Privacy

This appendix is informative.

The W3C TAG is developing a Self-Review Questionnaire: Security and Privacy for editors of specifications to informatively answer.

Per the Questions to Consider

  1. Does this specification deal with personally-identifiable information?


  2. Does this specification deal with high-value data?

    No. Credit card information and the like is not stored in WOFF2.

  3. Does this specification introduce new state for an origin that persists across browsing sessions?

    No. Specifically, WOFF2 fonts are only available to the documents that reference them; they do not persist for use by other applications or documents on the user's system.

  4. Does this specification expose persistent, cross-origin state to the web?


  5. Does this specification expose any other data to an origin that it doesn’t currently have access to?


  6. Does this specification enable new script execution/loading mechanisms?

    No. WOFF2 fonts do not contain, or link to, scripts.

  7. Does this specification allow an origin access to a user’s location?


  8. Does this specification allow an origin access to sensors on a user’s device?


  9. Does this specification allow an origin access to aspects of a user’s local computing environment?

    No. WOFF2 fonts are typically generated and stored on a server, and have no access to the user environment when generated. Once decompressed, they contain no active code and cannot inspect the user environment.

  10. Does this specification allow an origin access to other devices?


  11. Does this specification allow an origin some measure of control over a user agent’s native UI?


  12. Does this specification expose temporary identifiers to the web?


  13. Does this specification distinguish between behavior in first-party and third-party contexts?


  14. How should this specification work in the context of a user agent’s "incognito" mode?

    No differently.

  15. Does this specification persist data to a user’s local device?


  16. Does this specification have a "Security Considerations" and "Privacy Considerations" section?


  17. Does this specification allow downgrading default security characteristics?


The Security section of the font top level type [RFC8081] contains additional security-related information relevant to WOFF2.

WOFF2 uses Brotli compression. The Security Considerations section of the Brotli specification [Brotli] should be consulted, in addition to this appendix.

D. References

D.1 Normative References

WOFF File Format 1.0 Jonathan Kew, Tal Leming, Erik van Blokland, Editors. World Wide Web Consortium, 13 December 2012. The latest version of the WOFF specification is available at http://www.w3.org/TR/WOFF/
Open Font Format specification (ISO/IEC 14496-22:2015). together with Amendment 1: Updates for font collections functionality and Amendment 2: Updated text layout features and implementations
Brotli Compressed Data Format, RFC 7932 J. Alakuijala and Z. Szabadka, Editors. Internet Engineering Task Force, Informational, July 2016.
RFC 2119: Key words for use in RFCs to Indicate Requirement Levels. S. Bradner, Editor. Internet Engineering Task Force, Best Current practice, March 1997.
The font Top Level Type, RFC 8081 C. Lilley, Editor. Internet Engineering Task Force, Proposed Standard, February 2017.
CSS Fonts Module Level 3, John Daggett, Editor. World Wide Web Consortium Candidate Recommendation, 2013. The latest version of CSS3 Fonts is available at http://www.w3.org/TR/css-fonts-3/

D.2 Informative References

WOFF 2.0 Evaluation Report Chris Lilley, Editor. World Wide Web Consortium, 2015.
Microsoft OpenType specification. OpenType is a registered trademark of Microsoft Corporation.
Apple TrueType Reference manual. Apple, 2014. TrueType is a registered trademark of Apple, Inc.
MicroType® Express (MTX) Font Format Sarah Martin, Al Ristow, Steve Martin, Vladimir Levantovsky, and Christopher Chapman. W3C Member Submission, 5 March 2008.