WOFF File Format 2.0 (original) (raw)

Abstract

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 http://www.w3.org/TR/.

This is a Working Draft of the WOFF 2.0 specification. This document was developed by the WebFonts Working Group. The Working Group expects to advance this Working Draft to Recommendation Status. 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.

Please send comments about this document to www-font@w3.org (with public archive).

Publication as a Working Draft 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 5 February 2004 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.

This document is governed by the 1 August 2014 W3C Process Document.

Table of Contents

1. Introduction
1.1. Notational Conventions
2. General Requirements
3. Overall file structure and basic data types
3.1. Data types
3.2. WOFF2 Header
4. Font directory
4.1. Table directory format
4.2. Collection directory format
5. Compressed data format
5.1. Transformed glyf table format
5.2. Decoding of variable-length X and Y coordinates
5.3. Transformed loca table format
5.4. Table order constraints
6. Extended Metadata Block
7. Private Data Block
Appendix A: Internet Media Type Registration
Appendix B: Changes

References

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 [ 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.

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 are required 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 tags in the table directory in ascending alphabetical order 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, it differs in some important respects 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, . If such extraneous data is present . 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.

3.1. Data types

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 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, 203], [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.

Editor's note: We need to figure out how these conditions can be tested. Otherwise, we may need to edit statements that cannot be covered by the conformance tests and make them plain English descriptions instead.

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.

The interpretation of the WOFF2 Header is the same as the WOFF Header in [WOFF1]. 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.

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 consist 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 can be identified either by the known table tag as part of the flags field or by the 4-byte table tag, if present.

Contrary to the way how table directory entries are specified in the original input font file (where table directory entires 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 transofrm (see subclause 5.1 for details) is being applied to the 'glyf' table.

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

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 are reserved for future extensions.

Whether a table tag is encoded with a known table tag or explicitly including the four-byte tag has no semantic 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 is not a normative statement about whether such tags should be included in web fonts.

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.

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

Following the flags (and the optional tag field) are one to 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. Optionally, for 'glyf' and 'loca' tables that are subjected to additional transformations, the transformLength specifies the length of the transformed version of the table.

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 oulines point 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 tables are 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 folowed by an array of Collection Font Entries.

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:

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 physical tables both in an input font collection file and in the WOFF2 compressed data stream MUST be present only once and 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 encoderMUST 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 within 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 .

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.

Retention of font order in collection is required for CSS fragment identifiers to ever work.

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 table directory. 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 and loca 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. If the table is transformed, then 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 transofrms applied to glyf and loca tables, the decoder MUST recalculate the checkSumAdjustmentglyf 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 presense of the 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 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 descripbed above, the WOFF2 file MUST NOT be loaded.

A transform MUST be applied to two types of tables: glyf, representing outline data, and loca, representing the offsets of the individual glyphs within a glyf table, if these tables are present in a font. An authoring tool must transform the glyf and loca tables. Additional constraints apply, as specified in subclause 5.4. The glyf table transformation is specified in subclause 5.1, and the loca table transformation is specified in subclause 5.3.

Editor's note: Do we need to add the conformance requirement for UA regarding table transforms?

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 e.g. 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 transformation is intended to reduce redundant information and provide a more efficient encoding of the actual TrueType outlines of glyphs. The modified transformation is specified below and 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.

For greater compression effectiveness, the glyf table is split into seven substreams, to group like data together. The transformed table consists of a number of fields speciffying 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.

The format is best characterized by describing the decoding process, especially indications of what are valid and invalid data. An encoder MUST produce transformed data that is valid, it is up to the encoder to produce transformed data that decodes to the desired font 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 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 * ((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 MUST store for each glyph the corresponding offset in the 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 emtpy 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) involves creating a glyph record with no outlines where all bounding box values are equal to zero. If the bboxBitmap flag indicates that the bounding box values are to be inferred from the outline point coordinates, the decoder MUST generate the bounding box with xmin, xmax, ymin and ymax values equal to zero. If the bboxBitmap flag indicates that the bounding box values are explicitly encoded in the bboxStream the decoder MUST check that the encoded values are all equal to zero and, if they are not, 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 specificed 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 (1 << 5) set, go back to step 2a.

3a. If any of the flag words had the FLAG_WE_HAVE_INSTRUCTIONS bit (1 << 8) set, then read the instructions from the glyph and store them in the reconstructed glyph, using the same process as described in steps 5 and 6 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:

• Byte count (total number of bytes used for this set of coordinate values including one byte for 'flag' value).
• Number of bits used to represent X coordinate value (X bits).
• Number of bits used to represent Y coordinate value (Y bits).
• An additional incremental amount to be added to X bits value (delta X).
• An additional incremental amount to be added to Y bits value (delta Y).
• The sign of X coordinate value (X sign).
• The sign of Y coordinate value (Y sign).

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.

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

5.3. Transformed loca table format

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. 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. For a font with TrueType outlines, the glyf table MUST be encoded with the transform, which results in significantly smaller file size.

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.

In addition to table order, there is an additional constraint for transformedglyf tables: the origLength field MUST specify an adequate amount of space to represent the reconstructed glyf table. Typically, the origLength field value would specify the actual glyf table size; however, since there may be multiple valid reconstructions of a glyph record, according to the encoding rules specified in subclause 5.3.3 of [OFF], 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 glyf table may be greater than, less than, or equal to the actual size of the glyf table in the source font compressed by WOFF2. It is the responsibility of WOFF2 implementations to make sure that an adequate amout of memory is allocated when glyph records are being reconstructed from the WOFF2 file.

Editor's note: Is this really a testable assertion? How one can test for an "adequate amount of space" if the decoding process allows multiple permutations and all of them are equally valid, as far as UA and font rendering engines are concerned. Should we simplify this by saying that origLength is exactly what it means - the original length of the glyf table?

Regardless of the relation between original glyf table size and the size of the reconstructed table, an encoder MUST supply an origLength value for the transformed glyf table.

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 it MUST be stored compressed with [Brotli]. 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.

If present, the metadata MUST be compressed, in a separate Brotli stream from the main font data; it is never stored in uncompressed form.

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.

Appendix A: Internet Media Type Registration

This appendix registers a new top-level MIME media type, in conformance with BCP 13 and W3CRegMedia.

1. Abstract

This document serves to register and document the top-level MIME type for fonts, under which the subtype representation formats for fonts may be registered. This document also serves as a registration application for a set of intended subtypes, which are representative of some existing subtypes already registered under "application" tree by their separate registrations.

2. Introduction

The process of setting type in computer systems and other forms of text presentation systems uses fonts in order to provide visual representations of the glyphs. Just as with images, for example, there are a number of ways to represent the visual information of the glyphs. Early font formats often used bitmaps, as these could have been carefully tuned for maximum readability at a given size on low-resolution displays. More recently, scalable vector outline fonts have come into wide-spread use: in these fonts, the outlines of the glyphs are described, and the presentation system renders the outline in the desired position and size.

This document defines a top-level MIME type "font" under which different representation formats of fonts may be registered (e.g. a bitmap or outline formats). It should be emphasized that, just as under the "image" top-level type one does not find registration for a specific image, for example, "The Night-watch" (by Rembrandt) but instead "JPEG" (a compressed image data representation format), so, under "font" one will not find "Courier" (the name of a popular font) but perhaps "TTF", "OTF" or "SFNT" (the names of commonly used TrueType and OpenType font formats as well as their higher-level wrapper format).

3. Background and Justification

Historically there has not been a registration of formats for fonts. Most recently, there have been several representation formats registered as MIME subtypes under the "application" top-level type. However, with the rapid adoption of web fonts (based on the data from HTTP Archive showing a huge increase in web font usage from 1% in the end of 2010 to 50% across all sites in the beginning of 2015) custom fonts on the web have become a core web resource. As the in-depth analysis shows, the lack of the intuitive top-level font type is causing a significant confusion among developers - while currently defined font subtypes are severely under-utilized there are many more sites that already use non-existent (but highly intuitive) media types such as "font/woff", "font/ttf" and "font/truetype". At the same time, majority of sites resort to using generic types such as "application/octet-stream", "application/x-font-ttf", "text/plain" and "text/html", to name a few.

Contrary to our expectations, the officially defined IANA subtypes such as "application/font-woff" and "application/font-sfnt" see a very limited use - their adoption rates trail far behind as the actual use of web fonts continues to increase. The members of the W3C WebFonts WG believe the use of "application" top-level type is not ideal. First, the "application" sub-tree is treated (correctly) with great caution with respect to viruses and other active code. Secondly, the lack of a top-level type means that there is no opportunity to have a common set of optional attributes, such as are specified here. Third, fonts have a unique set of licensing and usage restrictions, which makes it worthwhile to identify this general category with a unique top-level type.

The W3C WebFonts WG believes that the situation can be significantly improved if a set of font media types is registered using "font" as a dedicated top-level type. Based on the data analysis presented above, we believe that it is the presense of simple and highly intuitive media types for images that caused the wide-spread adoption of IANA's recommendations, where the correct usage of existing media types reaches over 97% for all subtypes in the "image" tree. The WG believes that, considering a rapid adoption of fonts on the web, the registration of the top-level media type for fonts along with the intuitive set of subtypes that reflect popular and widely used data formats would further stimulate the adoption of web fonts, significantly simplify web server configuration process and facilitate the proper use of IANA media type recommendations.

4. Security considerations:

Fonts are interpreted data structures that represent collections of different tables containing data that represent different types of information, including glyph outlines in various formats, hinting instructions, metrics and layout information for multiple languages and writing systems, rules for glyph substitution and positioning, etc. Depending on the format used to represent the glyph data the font may contain TrueType, PostScript or SVG outlines and their respective hint instructions, where applicable. There are many existing, already standardized font table tags and formats that allow an unspecified number of entries containing predefined data fields for storage of variable length binary data. Many existing (TrueType, OpenType and OFF, SIL Graphite, WOFF, etc.) font formats are based on the table-based SFNT (scalable font) format which is extremely flexible, highly extensible and offers an opportunity to introduce additional table structures when needed, in a way that would not affect existing font rendering engines and text layout implementations. However, this very extensibility may present specific security concerns – the flexibility and ease of adding new data structures makes it easy for any arbitrary data to be hidden inside a font file. There is a significant risk that the flexibility of font data structures may be exploited to hide malicious binary content disguised as a font data component.

Fonts may contain 'hints', which are programmatic instructions that are executed by the font engine for the alignment of graphical elements of glyph outlines with the target display pixel grid. Depending on the font technology utilized in the creation of a font these hints may represent active code interpreted and executed by the font rasterizer. Even though hints operate within the confines of the glyph outline conversion system and have no access outside the font rendering engine, hint instructions can be, however, quite complex, and a maliciously designed complex font could cause undue resource consumption (e.g. memory or CPU cycles) on a machine interpreting it. Indeed, fonts are sufficiently complex, and most (if not all) interpreters cannot be completely protected from malicious fonts without undue performance penalties.

Widespread use of fonts as necessary component of visual content presentation warrants that a careful attention should be given to security considerations whenever a font is either embedded into an electronic document or transmitted alongside media content as a linked resource. While many existing font formats provide certain levels of protection of data integrity (such mechanisms include e.g. checksums and digital signatures), font data formats provide neither privacy nor confidentiality protection internally; if needed, such protection should be provided externally.

5. Definition and encoding

The "font" as the primary media content type indicates that the content identified by it requires certain graphic subsystem such as font rendering engine (and, in some cases, text layout and shaping engine) to process font data, which in turn may require certain level of hardware capabilities such as certain levels of CPU performance and available memory. The "font" media type does not provide any specific information about the underlying data format and how the font information should be interpreted - the subtypes defined within a "font" tree will name the specific font formats. Unrecognized sub-types of "font" should be treated as "application/octet-stream". Implementations may pass unrecognized subtypes to a common font-handling system, if such system is available.

6. Defined subtypes

In this section the initial entries under the top-level 'font' MIME type are documented. They also serve as examples for future registrations.

6.1. Generic SFNT font type

Type name:

font

Subtype name:

sfnt

Required parameters:

None.

Optional parameters:

1. Name: Outlines

Value: TTF, CFF, SVG

This parameter can be used to specify the type of outlines supported by the font. Value "TTF" shall be used when a font resource contains glyph outlines in TrueType format, value "CFF" shall be used to identify fonts containing PostScript/CFF outlines, and value SVG shall be used to identify fonts that include SVG outlines. TTF, CFF or SVG outlines can be present in various combiniations in the same font file, therefore, multiple values for the same optional parameter may be defined.

1. Name: Layout

Value: OTF, AAT, SIL

This parameter identifies the type of implemented support for advanced text layout features. The predefined values "OTF", "AAT" and " SIL" respectively indicate support for OpenType text layout, Apple Advanced Typography or Graphite SIL. More than one shaping and layout mechanism may be supported by the same font file, therefore, multiple values for the same optional parameter may be defined.

Encoding considerations:

Binary.

Interoperability considerations:

As it was noted in the first paragraph of the "Security considerations" section, the same font format wrapper can be used to encode fonts with different types of glyph data represented as either TrueType or PostScript (CFF) outlines. Existing font rendering engines may not be able to process some of the particular outline formats, and downloading a font resource that contains unsupported glyph data format would result in inability of application to render and display text. Therefore, it would be extremely useful to clearly identify the format of the glyph outline data within a font using an optional parameter, and allow applications to make decisions about downloading a particular font resource sooner. Similar, another optional parameter is suggested to identify the type of text shaping and layout mechanism that is supported by a font. Please note that as new outline formats and text shaping mechanisms may be defined in the future, the set of allowed values for two optional parameters defined by this section may be extended.

Published specification:

ISO/IEC 14496-22 "Open Font Format" (OFF) specification being developed by ISO/IEC SC29/WG11.

Applications that use this media type:

Any and all applications that are able to create, edit or display textual media content.

Additional information:

Magic number(s):

The TrueType fonts and OFF / OpenType fonts containing TrueType outlines should use 0x00010000 as the 'sfnt' version number.

The OFF / OpenType fonts containing CFF data should use the tag 'OTTO' as 'sfnt' version number.

File extension(s):

Font file extensions used for OFF / OpenType fonts: .ttf, .otf

Typically, .ttf extension is only used for fonts containing TrueType outlines, while .otf extension can be used for any OpenType/OFF font, either with TrueType or CFF outlines.

Macintosh file type code(s):

(no code specified)

@font-face Format:

none.

Fragment Identifiers

none.

Person & email address to contact for further information:

Vladimir Levantovsky (vladimir.levantovsky@monotype.com).

Intended usage:

COMMON

Restrictions on usage:

None

Author:

The ISO/IEC 14496-22 "Open Font Format" specification is a product of the ISO/IEC JTC1 SC29/WG11.

Change controller:

The ISO/IEC has change control over this specification.

6.2. TTF font type

Type name:

font

Subtype name:

ttf

Required parameters:

None.

Optional parameters:

Name: Layout

Value: OTF, AAT, SIL

This parameter identifies the type of support mechanism for advanced text layout features. The predefined values "OTF", "AAT" and " SIL" respectively indicate support for OpenType text layout, Apple Advanced Typography or Graphite SIL. More than one shaping and layout mechanism may be supported by the same font file, therefore, multiple values for the same optional parameter may be defined.

Encoding considerations:

Binary.

Interoperability considerations:

As it was noted in the first paragraph of the "Security considerations" section, the same font format can be used to encode fonts supporting different types of outlines and/or text shaping and layout mechanisms. Existing font rendering engine implementations may not be able to process some of the particular layout table formats, and downloading a font resource that contains unsupported text shaping mechanism would result in inability of applications to display text properly. Therefore, it would be extremely useful to clearly identify the supported text shaping and layout data within a font using an optional parameter, and allow applications to make decisions about downloading a particular font resource sooner. Please note that as new text shaping mechanisms may be defined in the future, the set of allowed values for the optional parameter defined by this section may be extended.

Published specification:

ISO/IEC 14496-22 "Open Font Format" (OFF) specification being developed by ISO/IEC SC29/WG11.

Applications that use this media type:

Any and all applications that are able to create, edit or display textual media content.

Additional information:

Magic number(s):

The TrueType fonts and OFF / OpenType fonts containing TrueType outlines should use 0x00010000 as the 'sfnt' version number.

File extension(s):

Font file extensions used for TrueType / OFF / OpenType fonts: .ttf, .otf

Typically, .ttf extension is only used for fonts containing TrueType outlines, while .otf extension may be used for any OpenType/OFF font, either with TrueType or CFF outlines.

Macintosh file type code(s):

(no code specified)

@font-face Format:

truetype

Fragment Identifiers

none.

Person & email address to contact for further information:

Vladimir Levantovsky (vladimir.levantovsky@monotype.com).

Intended usage:

COMMON

Restrictions on usage:

None

Author:

The ISO/IEC 14496-22 "Open Font Format" specification is a product of the ISO/IEC JTC1 SC29/WG11.

Change controller:

The ISO/IEC has change control over this specification.

6.3. OTF font type

Type name:

font

Subtype name:

otf

Required parameters:

None.

Optional parameters:

Name: Outlines

Value: TTF, CFF, SVG

This parameter can be used to specify the type of outlines supported by the font. Value "TTF" shall be used when a font resource contains glyph outlines in TrueType format, value "CFF" shall be used to identify fonts containing PostScript/CFF outlines, and value SVG shall be used to identify fonts that include SVG outlines. TTF, CFF or SVG outlines can be present in various combiniations in the same font file, therefore, multiple values for the same optional parameter may be defined.

Encoding considerations:

Binary.

Interoperability considerations:

As it was noted in the first paragraph of the "Security considerations" section, the same font format can be used to encode fonts with different types of glyph data represented as either TrueType, PostScript (CFF) or SVG outlines. Existing font rendering engines may not be able to process some of the particular outline formats, and downloading a font resource that contains unsupported glyph data format would result in inability of application to render and display text. Therefore, it would be extremely useful to clearly identify the format of the glyph outline data within a font using an optional parameter, and allow applications to make decisions about downloading a particular font resource sooner. Please note that as new outline formats may be defined in the future, the set of allowed values for the optional parameter defined in this section may be extended.

Published specification:

ISO/IEC 14496-22 "Open Font Format" (OFF) specification being developed by ISO/IEC SC29/WG11.

Applications that use this media type:

Any and all applications that are able to create, edit or display textual media content.

Additional information:

Magic number(s):

The TrueType fonts and OFF / OpenType fonts containing TrueType outlines should use 0x00010000 as the 'sfnt' version number.

The OFF / OpenType fonts containing CFF data should use the tag 'OTTO' as 'sfnt' version number.

File extension(s):

Font file extensions used for OFF / OpenType fonts: .ttf, .otf

Typically, .ttf extension is only used for fonts containing TrueType outlines, while .otf extension can be used for any OpenType/OFF font, either with TrueType, CFF or SVG outlines.

Macintosh file type code(s):

(no code specified)

@font-face Format:

opentype

Fragment Identifiers

none.

Person & email address to contact for further information:

Vladimir Levantovsky (vladimir.levantovsky@monotype.com).

Intended usage:

COMMON

Restrictions on usage:

None

Author:

The ISO/IEC 14496-22 "Open Font Format" specification is a product of the ISO/IEC JTC1 SC29/WG11.

Change controller:

The ISO/IEC has change control over this specification.

6.4. WOFF 1.0

Type name:

font

Subtype name:

woff

Required parameters:

None.

Optional parameters:

None.

Encoding considerations:

Binary.

Interoperability considerations:

None.

Published specification:

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

Applications that use this media type:

WOFF 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" 0x774F4646

File extension(s):

woff

Macintosh file type code(s):

(no code specified)

Macintosh Universal Type Identifier code:

org.w3c.woff

@font-face Format:

woff

Fragment Identifiers

none.

Person & email address to contact for further information:

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

Intended usage:

COMMON

Restrictions on usage:

None

Author:

The WOFF 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.

6.5. WOFF 2.0

Type name:

font

Subtype name:

woff2

Required parameters:

None.

Optional parameters:

None.

Encoding considerations:

Binary.

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):

woff2

Macintosh file type code(s):

(no code specified)

Macintosh Universal Type Identifier code:

org.w3c.woff2

@font-face Format:

woff2

Fragment Identifiers

none.

Person & email address to contact for further information:

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

Intended usage:

COMMON

Restrictions on usage:

None

Author:

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.

Appendix B: Changes

The following changes have been made, relative to the First Public Working Draft published on May 8, 2014.

• Clarified that the spec madates table directory sorting, while the table order itself should follow the OpenType/OFF recommended order.
• Added requirements for decoders to accept any possible encoding variations for 255UInt16 data type, recommend that encoders produced shorter 255UInt16 byte sequences whenever possible.
• Replaced the table-based descirption of the 255UInt16 data type with pseudo-code.
• Added conformance requirements for UIntBase128 data type.
• Added conformance requirements to WOFF 2.0 Header description mandating the reserved value to be set to zero.
• Added recommendation and conformance requirement for WOFF 2.0 encoders to remove DSIG table and set the flag (bit 11) of the font 'head' table.
• Added conformance requirement for decoders to store glyph record offsets in compliance with the specified indexFormat.
• Added conformance requirements to mandate implicit glyph bounding box calculations.
• Clarified the "Transformed 'glyf' table" content adding explicit references to every data stream and their overall order.
• Modified the "Transformed 'glyf' table" subclause adding detailed description of the glyph reconstruction process for empty, simple and composite glyphs.
• Added support for font collections in WOFF 2.0. As part of font collection support, added new requirement to encode 'glyf' and 'loca' tables as pairs, with 'loca' immediately following the 'glyf' table.
• Removed the notion of "nominal size" for decompressed WOFF 2.0 font since the font size can vary dramatically based on applied data optimization steps.
• Clarified the meaning of the uncompressed font data size.
• Clarified the differences in bounding box calculations for simple and composite glyphs.
• Added 4-byte alignment requirements for Extended Metadata and Private data blocks to match WOFF 1.0 Recommendation requirements.
• Added new appendix A with the draft text of top-level media type registration for a new 'font' type.
• Added this "Changes" appendix.

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 (this document) is available.

References

Normative References

[WOFF1]

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/

[OFF]

Open Font Format specification (ISO/IEC 14496-22:2009).

[Brotli]

Brotli Compressed Data Format, draft-alakuijala-brotli-02 J. Alakuijala and Z. Szabadka, October 2014.

[RFC2119]

RFC 2119: Key words for use in RFCs to Indicate Requirement Levels. S. Bradner, Editor. Internet Engineering Task Force, March 1997.

Informative References

[WOFF2ER]

WOFF 2.0 Evaluation Report Chris Lilley, Editor. World Wide Web Consortium, 2015.

[OpenType]

Microsoft OpenType specification. OpenType is a registered trademark of Microsoft Corporation.

[TrueType]

Apple TrueType Reference manual. Apple, 2002. TrueType is a registered trademark of Apple, Inc.

[MTX]

MicroType® Express (MTX) Font Format Sarah Martin, Al Ristow, Steve Martin, Vladimir Levantovsky, and Christopher Chapman. W3C Member Submission, 5 March 2008.

[CSS3-Fonts]

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/