Incremental Font Transfer

W3C Working Draft,

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This version:
https://www.w3.org/TR/2022/WD-IFT-20220628/
Latest published version:
https://www.w3.org/TR/IFT/
Editor's Draft:
https://w3c.github.io/IFT/Overview.html
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https://www.w3.org/standards/history/IFT
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Editors:
Chris Lilley (W3C)
(Apple Inc.)
(Google Inc.)

Abstract

This specification defines two methods to incrementally transfer fonts from server to client. Incremental transfer allows clients to load only the portions of the font they actually need which speeds up font loads and reduces data transfer needed to load the fonts. A font can be loaded over multiple requests where each request incrementally adds additional data.

Status of this document

This section describes the status of this document at the time of its publication. 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 produced by the Web Fonts Working Group as a Working Draft using the Recommendation track. This document is intended to become a W3C Recommendation.

If you wish to make comments regarding this document, please file an issue on the specification repository.

Publication as a Working Draft does not imply endorsement by W3C and its Members. 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 groups 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.

This document is governed by the 2 November 2021 W3C Process Document.

1. Introduction

This section is not normative.

Incremental Font Transfer (IFT) is a collection of technologies to improve the latency of remote fonts (or "web fonts") on the web. Without this technology, a browser needs to download every last byte of a font before it can render any characters using that font. IFT allows the browser to download only some of the bytes in the file, thereby decreasing the perceived latency between the time when a browser realizes it needs a font and when the necessary text can be rendered with that font.

The success of WebFonts is unevenly distributed. This specification allows WebFonts to be used where slow networks, very large fonts, or complex subsetting requirements currently preclude their use. For example, even using WOFF 2 [WOFF2], fonts for CJK languages are too large to be practical.

There are two different methods which can be used to incrementally transfer fonts.

1.1. Patch Subset

In the the first method, Patch Subset a server generates binary patches which a client applies to a subset of the font in order to extend the coverage of that subset. The server is stateless, it does not maintain any session data for clients between requests. Thus when a client requests the generation of a patch from the server it has to fully describe the current subset of the font that it has in a way which allows the server to recreate it.

Generic binary patch algorithms are used which do not need to be aware of the specifics of the font format. Typically a server will produce a patch by generating two font subsets: one which matches what the client currently has and one which matches the extended subset the client desires. A binary patch is then produced between the two subsets.

1.2. Range Request

The second method, Range Request, has no server-side requirements other than the server should be able to respond to byte-based range requests. The browser makes range requests to the server for the specific bytes in the font file that it needs. In order to know which bytes are necessary, the browser makes one initial special request for the beginning of the file to obtain all required font tables, and then calculates glyph coverage and required byte ranges using font character-to-glyph mapping and glyph substitution / layout tables.

In order for the range request method to be as effective as possible, the font file itself should be internally arranged in a particular way, in order to decrease the number of requests the browser needs to make. Therefore, it is expected that web developers wishing to use the range request method will use font files that have had their contents already arranged optimally.

This method was modelled after video playback on the web, where seeking in a video causes the browser to send a range request to the server.

1.3. Technical Motivation: Evaluation Report

See the Progressive Font Enrichment: Evaluation Report [PFE-report] for the investigation which led to this specification.

The evaluation report found that patch subset was generally more efficient in terms of overall performance and transferred bytes than range request. However, Range Request is simpler to deploy for many uses cases while still providing material improvments to loading performance for large fonts.

1.4. Performance Considerations and the use of Incremental Font Transfer

Using incremental transfer may not always be beneficial, depending on the characteristics of the font and the content being rendered. This section provides non-normative guidance to help decide:

  1. When incremental transfer should be utilized.

  2. When used, which of the two methods should be utilized.

Incrementally loading a font has a fundemental performance tradeoff versus loading the whole font. Simplistically, under incremental transfer less bytes may be transferred at the potential cost of increasing the total number of network requests being made, and/or increased request processing latency. In general incremental font transfer will be beneficial where the reduction in latency from sending less bytes outweighs additional latency introduced by the incremental transfer method.

The first factor to consider is the language of the content being rendered. The evaluation report contains the results of simulating incremental font transfer across three categories of languages (Progressive Font Enrichment: Evaluation Report § langtype). See it’s conclusions Progressive Font Enrichment: Evaluation Report § conclusions for a discussion of the anticipated performance of incremental font transfer across the language categories.

Next, how much of the font is expected to be needed? If it’s expected that most of the font will be needed to render the content then incremental font transfer is unlikely to be beneficial. In many cases however only part of a font is expected to be needed. For example:

An alternative to incremental transfer is to break a font into distinct subsets (typically by script) and use the unicode range feature of @font-face to load only the subsets needed. However, this can break rendering Progressive Font Enrichment: Evaluation Report § fail-subset if there are layout rules between characters in different subsets. Incremental font transfer does not suffer from this issue as it maintains the original font and all it’s layout rules.

1.4.1. Reducing the Number of Network Requests

As discussed in the previous section the most basic implementation of incremental font transfer will tend to increase the total number of requests made vs traditional font loading. Since each request will require at least one round trip time, performance can be negatively impacted if too many requests are made. Both range request and patch subset allow for more codepoints then needed to be requested. Intelligent use of this feature by an implementation can help reduce the total number of requests being made. The evaluation report explored this by testing the performance of a basic character frequency based codepoint prediction scheme and found it improved overall performance.

Performant implementations should incorporate a similar mechanism which can select codepoints which are likely to be needed in the future and preemptively load them. This will improve performance by reducing the need to make additional requests for missing codepoints. The set of codepoints the client has and the set they are requesting can be used in conjuction with codepoint occurence frequency and codepoint usage in languages/scripts to make predictions on which additional codepoints are likely to be needed.

Under range request the prediction mechanism will need to be part of the client implementation. In patch subset either the client and/or server implementation can include a prediction mechanism. A side benefit to a client side prediction mechanism is providing some obfuscation of the specific codepoints required by the client. This is further discussed in Content inference from character set.

1.4.2. Deciding between Patch Subset and Range Request

From a purely performance perspective patch subset is more performant than range request. There are three main factors for this:

  1. Range request requires an extra round trip on the very first load of a font to fetch the non-outline font data.

  2. Patch subset is able to incrementally transfer all tables in the font, while range request is limited to only incrementally transferring glyph outline tables. In many fonts the majority of data is in the glyph outlines, however there are fonts that have significant amounts of data in the non outline tables.

  3. Patch subset is able to compress incremental font data against previously loaded data from that font. This leads to better compression overall compared to range request.

See the evaluation report for a quantative assessment of the difference in performance of patch subset versus range request Progressive Font Enrichment: Evaluation Report § analysis.

The downside of using patch subset is that it requires server side processing to produce the patches, while range request can work on any standard HTTP server that supports range requests:

2. Opt-In Mechanism

This section is general to both IFT methods.

Webpage’s can choose to opt-in to either patch subset or range request incremental transfer for a font via the use of a CSS font tech keyword (CSS Fonts 4 § 11.1 Font tech) inside the @font-face block.

There are three tech keywords available:

The use of the incremental-patch keyword in this CSS rule indicates to the browser they should use the patch subset IFT method to load the font.
@font-face {
    font-family: "MyCoolWebFont";
    src: url("MyCoolWebFont.otf") tech(incremental-patch);
}

Note: Each individual @font-face block may or may not opt-in to IFT. This is due to the variety of ways fonts are used on web pages. Authors have control over which fonts they want to use this technology with, and which they do not.

Note: the use of incremental-auto may incur a CORS preflight request for the initial request of method negotiation, as the initial request sets a custom header.

Note: the IFT tech keywords can be used in conjuction with other font tech specifiers to perform font feature selection. For example a @font-face could include two urls one with tech(incremental-patch, color-COLRv1) and the other with tech(incremental-patch, color-COLRv0). The client would initiate an incremental patch subset transfer to one of the URLs depending on which version of COLR is supported.

3. IFT Method Selection

This section is general to both IFT methods.

The client should have support for both patch subset and range request IFT. The client selects the IFT method to utilize for a font load using the following procedure:

The most recently specified method by the content takes precedence. For example if on a site the first page specifies to use the patch subset method for a font URL, but the second page specifies range request for the same font URL then the second page should load the font using range request. Such cases will require the client to reset any previously stored state for that URL and start fresh with the new method. In the case that incremental-auto is encountered where previously a specific method was specified then the client can choose to continue using the previously selected method.

3.1. IFT Method Negotiation

When the particular IFT method(s) that are supported by a server are not known, the client must determine which method to use. Different clients may support different IFT methods, and different servers may support different IFT methods, so a negotation occurs as such:

  1. The browser makes the first request to the server using the GET HTTP method (HTTP Semantics § name-get). If the client prefers the patch-subset method, it sends the relevant patch request header. If the client prefers the range-request method, it does not send the header.

  2. If the server receives the patch request header and wishes to honor it, the server must reply with a valid patch subset response which includes the patch-subset magic number. Otherwise, the server must reply with the HTTP Semantics § range.requests Accept-Ranges header, if it supports HTTP Range Requests.

  3. If the client receives the patch-subset magic number, it commences using the patch-subset method. Otherwise, if the client receives the Accept-Ranges: bytes header, it commences using the range-request method. Otherwise, the whole font file is downloaded, and the current non-incremental loading behavior is used.

3.2. IFT Method Fallback

This section is not normative.

This summarizes behaviors that result from the above method selection and negotiation sections.

If the content specifies the range request method:

Client supports range-request method Client does not support range-request method
Server supports range-request method Range-request method is used. Client falls back to non-incremental load of the full font file.
Server does not support range-request method. Response is missing accept-ranges header. Client falls back to non-incremental load of the full font file. Client falls back to non-incremental load of the full font file.

If the content specifies the patch subset method:

Client supports patch-subset method Client does not support patch-subset method
Server supports patch-subset method Patch-subset method is used. Client falls back to non-incremental load of the full font file.
Server does not support patch-subset method Response is missing magic number. Client falls back to non-incremental load of the full font file. Client falls back to non-incremental load of the full font file.

If the content does not specify a specific method:

Client prefers range-request method Client prefers patch-subset method
Server supports both range-request method and patch-subset method Client makes initial request without the patch request header, and possibly with the Range header. Because all patch-subset servers must support the range-request method, the server replies with Accept-Ranges and initial font data. Client/server commence using range-request method. Client makes initial request with the patch request header. Server replies with the patch-subset magic number, and client/server commence using patch-subset method.
Server supports only range-request method Same as above. Client makes initial request with the patch request header. Server replies with Accept-Ranges and initial font data. Client/server commence using range-request method.
Server supports neither Client makes initial request without the patch request header, and possibly with the Range header. Server replies without Accept-Ranges header, and sends the full font file to the client from beginning to end. Client makes initial request to server with the patch request header. Server does not reply with the patch-subset magic number, and sends the full font file to the client from beginning to end.

3.3. Offline Usage

Special consideration must be taken when saving a page for offline usage that uses an incrementally transferred font since the saved page won’t be able to increment the font if content changes (eg. due to javascript execution). In these cases the page saving mechanism should download the full font by making a normal GET request without the patch request header to the font url. Additionally when URL’s are rewritten to point to the saved full font any of the incremental tech specifiers should be removed.

4. Patch Based Incremental Transfer

4.1. Font Subset

A font subset is a modified version of a font file that contains only the data needed to render a subset of:

supported by the original font. When a subsetted font is used to render text using any combination of the subset codepoints, layout features, or design-variation space it must render identically to the original font. This includes rendering with the use of any optional typographic features that a renderer may choose to use from the original font, such as hinting instructions.

A font subset definition describes the minimum data (codepoints, layout features, variation axis space) that a font subset must possess.

4.2. Data Types

This section lists all of the data types that are used to form the request and response messages sent between the client and server.

4.2.1. Encoding

All data types defined here are encoded into a byte representation for transport using CBOR (Concise Binary Object Representation) [RFC8949]. More information on how each data types should be encoded by CBOR are given in the definition of those data types.

4.2.2. Primitives

Data Type Description CBOR Major Type
Integer An integer value range [-263, 263 - 1] inclusive. 0 or 1
Float IEEE 754 Single-Precision Float. 7
ByteString Variable number of bytes. 2
String UTF-8 [rfc3629] text string. 3
ArrayOf<Type> Array of a variable number of items of Type. 4

4.2.3. ProtocolVersion

An Integer describing the version of this communication protocol being used by a PatchRequest or PatchResponse. This value guides the semantics and interpretation of the fields sent.

This field is for future expansion. There currently is only one valid value, 0.

4.2.4. SparseBitSet

A data structure which compactly stores a set of distinct unsigned integers. The set is represented as a tree where each node has a fixed number of children that recursively sub-divides an interval into equal partitions. A tree of height H with branching factor B can store set membership for integers in the interval [0 to BH-1] inclusive. The tree is encoded into a ByteString for transport.

To construct the tree T which encodes set S first select the branching factor B (how many children each node has). B can be 4, 8, 16, or 32.

Note: the encoder can use any of the possible branching factors, but it is recommended to use 4 as that has been shown to give the smallest encodings for most sets typically encountered.

Next, determine the height, H, of the tree:

H = ceil(logB(max(S) + 1))

If S is an empty set then H = 1.

Next create a tree of height H where all non-leaf nodes have B children. Each node in the tree has a single value composed of B bits. Given a node p which has B children: c0 ... cB - 1 and is in a tree, T, of height H, then:

The tree is encoded into a bit string. When appending multiple-bit values to the bit string, bits are added in order from least significant bit to most significant bit.

First append 2 bits which encode the branching factor:

Bits  Branching Factor
00 2
01 4
10 8
11 32

Then append the value H - 1 as a 5 bit unsigned integer. Next append a single 0 bit, which is reserved for future use.

Next the nodes are encoded into the bit string by traversing the nodes of the T in level order and appending the value for each non-zero node to the bit string. If all of the set values covered by a node’s interval are present within set S, then that node can instead be encoded in the bit string as B bits all set to zero. All children of that node must not be encoded.

Lastly the bit string is converted into a ByteString by converting each consecutive group of 8 bits into the next byte of the string. If the number of bits in the bit string is not a multiple of 8, zero bits are appended to the next multiple of 8. The bit with the smallest index in the bit string is the least significant bit in the byte and the bit with the largest index is the most significant bit.

The set {2, 33, 323} in a tree with a branching factor of 8 is encoded as the bit string:
BitString:
|- header |- lvl 0 |---- level 1 ----|------- level 2 -----------|
|         |   n0   |   n1       n2   |   n3       n4       n5    |
[ 01010000 10000100 10001000 10000000 00100000 01000000 00010000 ]

Which then becomes the ByteString:
[
  0b00001010,
  0b00100001,
  0b00010001,
  0b00000001,
  0b00000100,
  0b00000010,
  0b00001000
]

First determine the height of the tree:

H = ceil(log8(323 + 1)) = 3

Then append

Level 0:

Level 1:

Level 2:

The set {} in a tree with a branching factor of 4 is encoded as the bit string:
BitString:
|- header- |
|          |
[ 00000000 ]

Which then becomes the ByteString:
[
  0b00000000,
]

First determine the height of the tree. Because we are encoding an empty set height is:

H = 1

Then append

Empty sets have no nodes, so no bytes beyond the header need to be appended.

The set {0, 1, 2, ..., 17} can be encoded with a branching factor of 4 as:
BitString:
|- header | l0 |- lvl 1 -| l2  |
|         | n0 | n1 | n2 | n3  |
[ 10010000 1100 0000 1000 1100 ]

ByteString:
[
  0b00001001,
  0b00000011,
  0b00110001
]

First determine the height of the tree:

H = ceil(log4(17 + 1)) = 3

Then append

Level 0:

Level 1:

Level 2:

4.2.5. IntegerList

A data structure which compactly represents a list of non-negative integers from 0 to 231-1. The list is encoded into a ByteString for transport.

There are three steps of encoding/compression: first delta, second zig-zag, and finally UIntBase128. The final ByteString result is simply the concatenation of the individual UIntBase128 encoded bytes.

IntegerList encoding must reject an input list which contains values not in the range 0 to 231-1. Likewise if decoding an IntegerList results in values which are not in the range 0 to 231-1 the list is invalid and must be rejected.

4.2.5.1. Delta Encoding

Delta encoding converts a list of integers to a list of deltas between them.

A list L of n integers Li0..n-1 is converted into a list of N integers Di0..n-1 as follows:

This has the effect of reducing the magnitude of the values, which reduces the number of bytes required in the UIntBase128 encoding, below.

// Note: unsorted
int_list = [23, 43, 12, 3, 67, 68, 69, 0]
delta_list = [23, 20, -31, -9, 64, 1, 1, -69]
4.2.5.2. Zig-Zag Encoding

Zig-Zag encoding reversibly converts signed integers to unsigned integers, using the same number of bits. The entire range of values is supported. This step is required, as the § 4.2.5.3 UIntBase128 Encoding step works on unsigned integers only. The encoding maps positive integer values to even positive integers and negative integer values to odd positive integers. Psuedo code:

encode(n):
  if n >= 0:
    return n * 2
  else:
    return (n * -2) - 1

decode(n) {
  if n & 1:
    return -((n + 1) / 2)
  else:
    return n / 2
Value Zig-Zag Encoding
0 0
1 2
2 4
3 6
4 8
-1 1
-2 3
-3 5
-4 7
delta_list = [23, 20, -31, -9, 64, 1, 1, -69]
zig_zag_encoded_list = [46, 40, 61, 17, 128, 2, 2, 137]
4.2.5.3. UIntBase128 Encoding

UIntBase128 is a 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 response/request 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. Pseudo-code:

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;
}
Value       Output Bytes
0           00000000
1           00000001
2           00000010
3           00000011
127         01111111
128         10000001 00000000
255         10000001 01111111
16256       11111111 00000000
2080768     11111111 10000000 00000000
266338304   11111111 10000000 10000000 00000000
4294967295  10001111 11111111 11111111 11111111 01111111
zig_zag_encoded_list = [46, 40, 61, 17, 128, 2, 2, 137]
bytes = [2E 28 3D 11 81 00 02 02 81 09]
         └┘ └┘ └┘ └┘ └───┘ └┘ └┘ └───┘

4.2.6. SortedIntegerList

A data structure which compactly represents a sorted list of ascending non-negative integers (0 to 232-1). The list is encoded into a ByteString for transport.

This is a variation on IntegerList with better compression. Sorted lists only use two steps of encoding/compression: first deltas and then UIntBase128. The § 4.2.5.2 Zig-Zag Encoding step is skipped. This allows twice the range in UIntBase128, so that single bytes may be used more often.

SortedIntegerList encoding must reject an input list which contains values not in the range 0 to 232-1. Likewise if decoding an IntegerList results in values which are not in the range 0 to 232-1 the list is invalid and must be rejected.

4.2.7. RangeList

A RangeList encodes a set of non-negative integers (0 to 232-1). The set is encoded as a list of disjoint intervals. Each interval is represented by two integers, a start (inclusive) and end (inclusive).

A RangeList is a list of n pairs [mini0..n-1, maxi0..n-1]. The list must be non-decreasing, i.e. mini=1..n-1 >= maxi-1.

To encode this list, we convert it to a list L of 2n integers, where L2i = mini and L2i+1 = maxi for i = 0..n-1.

L is a sorted list of integers, so a SortedIntegerList is used to encode it as a ByteString.

range_list = [3, 10], [13, 268]
int_list = [3, 10, 13, 268]
delta_list = [3, 7, 3, 255]
bytes = [03 07 03 81 7F]

4.2.8. FeatureTagSet

A FeatureTagSet encodes a set of zero or more opentype layout feature tags. Each feature tag is mapped to an integer value and then the set of mapped integers are encoded in a SortedIntegerList. Feature tags are mapped to integers as follows:

The final encoding is produced by sorting the mapped integers (exlcuding tags which are skipped) into ascending order and then encoding the sorted list as a SortedIntegerList.

When decoding a FeatureTagSet the integer values are mapped back to the original tags by reversing the above mapping rules. Additionally all default features in Appendix B: Default Feature Tags and Encoding IDs must be added to the decoded set.

4.2.9. AxisSpace

Stores a set of intervals on one or more open type variation axes [opentype-variations]. Encoded as a CBOR map (major type 5). The key in each pair is an axis tag. It is encoded as a ByteString containing exactly 4 ASCII characters. The value in each pair is an ArrayOf<AxisInterval>. The list of intervals for a each axis tag must be disjoint.

4.2.10. Objects

Objects are data structures comprised of key and value pairs. Objects are encoded via CBOR as maps (major type 5). Each key and value pair is encoded as a single map entry. Keys are always unsigned integers and are encoded using major type 0. Values are encoded using the encoding specified by the type of the value.

All fields in an object are optional and do not need to have an associated value. Conversely when decoding and object fields may be present which are not specified in the schema. The decoder must ignore without error any key and value pairs where the key is not recognized.

There are several types of object used, each type is defined by a schema in § 4.3 Object Schemas. The schema for a type specifies for each field:

4.3. Object Schemas

4.3.1. CompressedSet

Encodes a set of unsigned integers. The set is not ordered and does not allow duplicates. Members of the set are encoded into either a SparseBitSet or a RangeList. To obtain the final set the members of sparse_bit_set and the list of ranges in range_deltas are unioned together.

ID  Field Name Type
0 sparse_bit_set SparseBitSet (ByteString)
1 range_deltas RangeList (ByteString)

4.3.2. AxisInterval

ID Field Name Value Type
0 start Float
1 end Float

AxisInterval defines an interval (from start to end inclusive) on some variable axis in a font.

For an AxisInterval object to be well formed:

4.3.3. PatchRequest

ID Field Name Value Type
0 protocol_version ProtocolVersion (Integer)
1 accept_patch_format ArrayOf<Integer>
2 codepoints_have CompressedSet
3 codepoints_needed CompressedSet
4 indices_have CompressedSet
5 indices_needed CompressedSet
6 features_have FeatureTagSet
7 features_needed FeatureTagSet
8 axis_space_have AxisSpace
9 axis_space_needed AxisSpace
10 ordering_checksum Integer
11 original_font_checksum Integer
12 base_checksum Integer
13 fragment_id String

For a PatchRequest object to be well formed:

4.3.4. PatchResponse

ID Field Name Value Type
0 protocol_version ProtocolVersion (Integer)
1 patch_format Integer
2 patch ByteString
3 replacement ByteString
4 original_font_checksum Integer
5 codepoint_ordering IntegerList
6 subset_axis_space AxisSpace
7 original_axis_space AxisSpace
8 original_features FeatureTagSet

For a PatchResponse object to be well formed:

4.4. Client

4.4.1. Client State

The client will need to maintain at minimum the following state for each font file being incrementally transferred:

Additionally, the client can optionally store:

4.4.2. Extending the Font Subset

This algorithm is used by the client to extends its font subset to cover additional codepoints, features, and/or design-variation space.

Extend the font subset

The inputs to this algorithm are:

The algorithm outputs:

The algorithm:

  1. Compare the desired subset definition to the client state. If the font subset in the client state already satisifies the font subset definition. Then return client state, and null for the cache fields.

  2. Otherwise make an HTTP request using the fetch algorithm:

    • The request method must be either "GET" or "POST".

    • The request destination must be "font".

    • The request CORS mode must be "cors".

    • The request cache mode should be "no-store".

    • The request URL scheme must be "https".

    • The request URL path is set to the input font URL.

    • If method is "POST" then, request body must be a single PatchRequest object encoded via CBOR.

    • Otherwise if method is "GET" then, a header with name Font-Patch-Request (the patch request header) and whose value is a single PatchRequest object encoded via CBOR and then base64url encoding [rfc4648] must be added to the request’s header list.

    Any request and/or url parameters which are not specified here should be set based on the user agent’s normal handling for font requests. For example if this font load is from a CSS font face, then CSS Fonts 4 § 4.8.2 Font fetching requirements should be followed.

    The fields of the PatchRequest object should be set as follows:

    Note: It is allowed for the client to request more codepoints then it strictly needs. For example, on slower connections it may be more performant to request extra codepoints if that is likely to prevent a future request from needing to be sent.

  3. Invoke Handle server response with the response from the server and return the result.

Note: POST is preferred for the HTTP method since it will not cause a CORS preflight request and the request object is more compactly encoded. GET should only be used during method negotiation.

4.4.3. Handling Server Response

If a server is able to succsessfully process a PatchRequest it will respond with HTTP status code 200 and the body of the response will be a 4 byte magic number (0x49, 0x46, 0x54, 0x20) followed by a single PatchResponse object encoded via CBOR. The client uses the following algorithm to interpret and process the fields of the object.

Handle server response

Inputs:

The algorithm outputs:

The algorithm:

  1. If the server response has status other than 200:

  2. Check the first four bytes of the server response body. If they are not equal to [0x49, 0x46, 0x54, 0x20] then this response is malformed. Invoke Handle failed font load and return the result.

  3. Interpret the remaining bytes of the server response body as a CBOR encoded PatchResponse. If the bytes are not decodable with CBOR, or the PatchResponse is not well formed, then this is an error. Invoke Handle failed font load and return the result.

  4. If field replacement is set then: the byte array in this field is a binary patch in the format specified by patch_format. Apply the binary patch to a base which is an empty byte array. Replace the font subset in the input client state with the result of the patch application.

  5. If field patch is set then: the byte array in this field is a binary patch in the format specifiedy patch_format. Apply the binary patch to the font subset from the input client state. Replace the font subset in the input client state with the result of the patch application.

  6. If original_font_checksum is set then update the original font checksum in the input client state with the value in original_font_checksum.

  7. If the field codepoint_ordering is set then update the codepoint reordering map in the input client state with the new values specified the field. If neither replacement nor patch are set, then the client should resend the request that triggered this response but use the new codepoint ordering provided in this response. Go back to step 1 to process the response.

  8. If original_features is set and the client has opted to save them then replace the original font feature list in the input client state with the value from the response.

  9. If original_axis_space is set then update the original font axis space in the input client state with the value specified in this field.

  10. If subset_axis_space is set then update the subset axis space in the input client state with the value specified in this field.

After processing the response, return the updated input client state and any cache headers that were set on the server response.

Handle failed font load

If the font load or extension has failed the client should choose one of the following options:

  1. If the client has a saved font subset, it may choose to use that and then use the user agent’s existing font fallback mechanism for codepoints not covered by the subset.

  2. The client may re-issue the request as a regular non incremental font fetch to the same path. It must not include the patch subset request parameter or header. This will load the entire original font.

  3. Discard the saved font subset, and use the user agent’s existing font fallback mechanism.

Regardless of which of the above options are used, the saved client state for this font must be discarded.

4.4.4. Load a Font in a User Agent with a HTTP Cache

The previous section § 4.4.2 Extending the Font Subset provides no guidance on how a user agent should handle saving client state between invocations of the subset extension algorithm. This section provides an algorithm that user agents which implement [fetch] should use to save client state to the user agent’s HTTP cache ([RFC9111]).

Load a font with a HTTP Cache

The inputs to this algorithm:

The algorithm outputs:

The algorithm:

  1. Make a HTTP fetch:

    • The request method is "GET".

    • The request destination must be "font".

    • The request CORS mode must be "cors".

    • The request URL scheme must be "https".

    • The request URL path is set to the input font URL.

    • The request cache mode is "only-if-cached".

  2. If the request is successful and the response is "fresh" (HTTP Caching § name-freshness) then invoke Extend the font subset with:

    • Font url set to the input font URL.

    • Fragment identifier set to the input fragment identifier

    • Client state set to the response.

    • Desired subset definition set to the input desired subset definition.

    • Fetch algorithm set to the input fetch algorithm.

    Once that returns go to step 4.

  3. Otherwise, invoke Extend the font subset with:

    • Font url set to the input font URL.

    • Fragment identifier set to the input fragment identifier

    • Client state set to null.

    • Desired subset definition set to the input desired subset definition.

    • Fetch algorithm set to the input fetch algorithm.

    Once that returns go to step 4.

  4. If the returned cache fields are non-null update the cache entry for the input font url with the returned client state and returned cache fields.

  5. Return the font subset contained in the returned client state.

4.5. Server: Responding to a PatchRequest

If the server receives a well formed PatchRequest over HTTPS for a font the server has and that was populated according to the requirements in § 4.4.2 Extending the Font Subset then it must respond with HTTP status code 200. The first 4 bytes of the response body must be set to 0x49, 0x46, 0x54, 0x20 ("IFT " encoded as ASCII) followed by a single PatchResponse object encoded via CBOR.

The path in the request url identifies the specific font that a patch is desired for. If the request has the fragment_id field set and the file identified by path is a collection of fonts, then fragment_id identifies the font within that collection that a patch is desired for. The identified font is referred to as the original font in the rest of this section.

From the request object the server can produce two codepoint sets:

  1. Codepoints the client has: formed by the union of the codepoint sets specified by codepoints_have and indices_have. The indices in indices_have must be mapped to codepoints by the application of the codepoint reordering with a checksum matching ordering_checksum.

  2. Codepoints the client needs: formed by the union of the codepoint sets specified by codepoints_needed and indices_needed. The indices in indices_needed must be mapped to codepoints by the application of the codepoint reordering with a checksum matching ordering_checksum.

Likewise, the server can produce two sets of opentype layout feature tags:

  1. Feature tags the client’s subset has: specified by features_have. If the field is unset this indicates the client’s subset contains all features in the original font.

  2. Feature tags the client needs: specified by features_needed. If the field is unset this indicates the client wants all features in the original font.

Lastly, the server can produce two variable axis spaces:

  1. Axis space the client has: specified by axis_space_have. If any axes in the font are not specified in axis_space_have then for those axes add their entire interval from the original font.

  2. Axis space the client needs: specified by axis_space_needed. If any axes in the font are not specified in axis_space_needed then for those axes add their entire interval from the original font.

If the server does not recognize the codepoint ordering used by the client, it must respond with a response that will cause the client to update it’s codepoint ordering to one the server will recognize via the process described in § 4.4.3 Handling Server Response and not include any patch. That is the patch and replacement fields must not be set.

Otherwise when the response is applied by the client following the process in § 4.4.3 Handling Server Response to a font subset with checksum base_checksum it must result in an extended font subset:

If the checksum of the original font computed by the procedure in § 4.6 Computing Checksums does not match the checksum in original_font_checksum or original_font_checksum is unset, then:

Additionally:

Note: the server can respond with either a patch or a replacement but should try to produce a patch where possible. Replacement’s should only be used in situations where the server is unable to recreate the client’s state in order to generate a patch against it.

Note: if a patch subset service is composed of more than one server task and some subset of those tasks are using a subsetter version which produces different binary results than the rest, there is a risk that consecutive extend requests may result in unnecessary replacement responses. For example if consecutive requests alternate between server backends with different subsetters, then each response will be a replacement as the server tasks will be unable to recreate the previously generated subset. This scenario might occur during software updates to the server tasks. To combat this it’s recommended that sticky load balancing is used which aims to send consecutive requests from the same client to the same server task.

Possible error responses:

4.5.1. Range Request Support

A patch subset support server must also support incremental transfer via § 5 Range Request Incremental Transfer. To support range request incremental tranfser the patch subset server must support HTTP range requests (HTTP Semantics § range.requests) against the font files it provides via patch subset.

4.6. Computing Checksums

64 bit checksums of byte strings are computed using the [fast-hash] algorithm. A python like pseudo code version of the algorithm is presented below:

# Constant values come fast hash: https://github.com/ztanml/fast-hash
SEED = 0x11743e80f437ffe6
M = 0x880355f21e6d1965

mix(value):
  value = value ^ (value >> 23)
  value = value * 0x2127599bf4325c37
  value = value ^ (value >> 47)
  return value

fast_hash(byte[] data):
  # When casting byte arrays into unsigned 64 bit integers the bytes are in little
  # endian order. That is the smallest index is the least significant byte.
  uint64 hash = SEED ^ (length(data) * M)
  for (i = 0; i <= length(data) - 8; i += 8)
    hash = (hash ^ mix((uint64) data[i:i+8])) * M

  remaining = length(data) % 8
  if not remaining:
    return mix(hash)

  uint64 last_value = (uint64) concat(data[length(data) - remaining:],
                                      [0] * (8 - remaining))
  return mix((hash ^ mix(last_value)) * M)

To ensure checksums are consistent across all platforms, all integers during the computation are in little endian order.

Note: a C implementation of fast hash can be found here: [fast-hash]

Bytes Checksum value
0f 7b 5a e5 0xe5e0d1dc89eaa189
1d f4 02 5e d3 b8 43 21 3b ae de 0xb31e9c70768205fb

4.7. Codepoint Reordering

A codepoint reordering for a font defines a function which maps unicode codepoint values from the font to a continuous space of [0, number of codepoints in the font). This transformation is intended to reduce the cost of representing codepoint sets.

A codepoint ordering is encoded into a IntegerList. The list must contain all unicode codepoints that are supported by the font. The index of a particular unicode codepoint in the list is the new value for that codepoint.

A server is free to choose any codepoint ordering, but should try to pick one that will minimize the size of encoded codepoint sets for that font.

4.7.1. Codepoint Reordering Checksum

A checksum of a codepoint reordering can be computed as follows:

SEED = 0x11743e80f437ffe6
M = 0x880355f21e6d1965

mix(value):
  value = value ^ (value >> 23)
  value = value * 0x2127599bf4325c37
  value = value ^ (value >> 47)
  return value

fast_hash_ordering(uint64[] ordering):
  uint64 hash = SEED ^ (length(ordering) * 8 * M)
  for i in ordering:
    hash = (hash ^ mix(ordering[i])) * M

  return mix(hash)

To ensure checksums are consistent across all platforms, all integers during the computation are in little endian order.

Codepoint Ordering Checksum value
[106, 97, 105, 120, 100] 0x6986dc19f4e621e

4.8. Patch Formats

The following patch formats may be used by the server to create binary diffs between a source file and a target file:

Format Value Notes
VCDIFF 0 Uses VCDIFF format [RFC3284] to produce the patch. All client and server implementations must support this format.
Brotli Shared Dictionary 1 The target file is encoded with brotli compression using the source file as a shared LZ77 dictionary. If the source file is empty then the target file is just compressed using brotli compression with no shared dictionary.

5. Range Request Incremental Transfer

5.1. Introduction to Range Request

This section is not normative.

The Range Request method is a very simple method of incremental transfer, and has no server-side requirements (other than the server should be able to respond to byte-based range requests). The browser simply makes range requests to the server for the specific bytes in the font file that it needs. In order to know which bytes are necessary, the browser makes one initial special request for the beginning of the file.

In order for the range request method to be as effective as possible, the font file itself should be internally arranged in a particular way, in order to decrease the number of requests the browser needs to make. Therefore, it is expected that web developers wishing to use the range request method will use font files that have had their contents already arranged optimally.

This method was modelled after video playback on the web, where seeking in a video causes the browser to send a range request to the server.

5.2. Font organization

5.2.1. Background

This section is not normative.

A particular organization of font files is beneficial for improving the performance of the range-request IFT method. The range-request IFT method only works with [TRUETYPE], [OPENTYPE], [WOFF], or [WOFF2] files. All of these file formats use an sfnt wrapper which provides a directory of tables inside the font file. A sfnt-based font file is mainly composed of a collection of independent tables.

Using WOFF2 files with range-request mechanism doesn’t seem to be a viable option

5.2.2. Introduction

The term range-request optimized font is used to describe a font file organized for use with the range-request IFT method. Optimizing a font for the range-request IFT method does not change the file format of the font.

Note: Because optimizing a font does not change its file format, no new additional tooling is necessary to interact with these optimized fonts. They are still valid fonts, but with a particular internal organization.

The result of optimizing an OpenType font for the range-request IFT method is still a valid OpenType font. The resulting file may be larger (by byte count) than it was before optimizing it, but fewer of those bytes should be necessary for a client to download in order to render a target text.

Note: There are no MUST-level requirements on the organization of a range-request optimized font. Any arbitrary font file may be considered to be a range-request optimized font. However, additional optimizations should increase the performance of loading in a browser via the range-request IFT method. Font creators are encouraged to enact as many of the optimizations listed in this section as are reasonable for the fonts they create.

5.2.3. Compression

Servers supporting the range-request IFT method should support compression via the Content-Encoding header (HTTP Semantics § name-content-encoding), rather than having the font file itself be statically compressed.

A range-request optimized font file (the file itself) should not use any kind of compression other than [RFC7932] (commonly referred to as "Brotli") compression.

If Brotli compression is used in a range-request optimized font, it should use only one meta-block.

If Brotli compression is used in a range-request optimized font, its one meta-block should have the ISUNCOMPRESSED bit set to 1.

Static file compression compatibility with range-request method

5.2.4. Table Ordering

The term outline table is used to describe these three tables, which carry different types of glyph outlines:

A range-request optimized font should have only one outline table.

No two tables in a range-request optimized font should share a tag name.

The outline table data in a range-request optimized font should lie at the end of the file.

If a CFF table exists, the CharString data should lie at the end of the CFF table.

If a CFF2 table exists, the CharString data should lie at the end of the CFF2 table.

Font Collections support

5.2.5. Glyph Independence

Note: The goal of this section is to make every glyph independent from each other.

A range-request optimized font should not use Compound glyphs.

Supporting fonts with composite glyphs via range-request

Note: Compound glyphs can be flattened by inlining their component glyphs to become additional contours.

A range-request optimized font should not use Subroutines.

Note: CFF glyph CharStrings can be flattened by inlining subroutines to become additional CharString bytes.

5.2.6. Glyph Order

Glyphs inside a range-request optimized font should be sorted in the file to keep glyphs often used in the same documents close together in the file.

Note: Putting the most frequently used glyphs together in the font increases the likelihood that the browser can download a contiguous sequence of necessary glyphs in a single range-request, thereby minimizing overhead.

Note: Reordering glyphs in a font is the same conceptual operation as renaming glyphs to have different glyph IDs. Therefore, this operation cannot be completed if glyph IDs must be preserved. Because glyph IDs are internal to text processing procedures and are not persisted, this requirement is not expected to be particularly burdensome.

One suggested method of sorting the glyphs in the file is by usage document frequency inside a relevant corpus.

A corpus is defined to be a collection of documents, where a documents includes a collection of glyphs necessary to render some textual content.

Note: For a particular website, a corpus might be defined to be individual page loads of pages on that website.

The usage document frequency of a particular glyph inside a corpus is defined by the number of documents in the corpus which use that glyph, divided by the number of documents in the corpus.

Note: This is distinct, but similar, to the number of times the glyph is used throughout the entire corpus.

The usage document frequency of the glyphs in a range-request optimized font should be decreasing throughout the font; that is, the most frequently used glyphs should have the lowest glyph IDs.

Note: Glyph ID 0 cannot be renamed in OpenType, TrueType, WOFF, and WOFF 2 fonts. All other glyphs can be renamed.

Note: Because the goal is simply to minimize overhead by placing similarly-used glyphs together, it may actually be possible to do better than ordering by simple frequency for a particular corpus. For example, some corpuses may have cliques of glyphs which have different frequencies but which nevertheless always seem to be used together.

A suggested ordering is included in Appendix A below.

5.3. Browser Behaviors

5.3.1. First Request

When a browser encounters the CSS opt-in mechanism, it is instructed to use IFT to load the fonts. First, it follows the steps in the IFT method selection section above. If those steps result in using the range-request method, the rest of this section applies.

The IFT method selection involves a single round-trip to the server, and if the range-request method is being used, the server’s response starts sending the font file to the browser. The browser should start parsing the partial font data as it is being loaded from the server. The browser should not wait until the entire file has been received before parsing its contents.

There is a certain amount of data from the beginning of the font file which the browser should unconditionally download. The boundary at the end of this data is called the range-request threshold.

Note: The first request does not have to be a range request. If the browser expects the range-request threshold to lie within the first n bytes of the font, the first request may be a range request for the first n bytes of the font. However, a browser may instead make a non-range request, parse the data as it is being streamed from the server, and discover that it has reached the range-request threshold while data is still being streamed.

Once all the data before the range-request threshold has been loaded by the browser, the browser may either close this connection to the server, or it may choose to leave the connection open and let the font data continue loading in the background.

A browser may choose to add a Range header (HTTP Semantics § name-range) to the initial request during the IFT method selection if it has reason to believe the range it requests will be large enough and it prefers to not close this connection to the server.

Note: Different browsers may choose different range-request thresholds. Some browsers may treat this threshold as occuring at the end of the sfnt tableDirectory. Other browsers may treat this threshold as occurring just before any outline data, provided the outline data appears at the end of the font. Other browsers may place this threshold at the very beginning of the file, thereby treating the whole file as able to be downloaded with range-requests.

5.3.2. Subsequent Requests

After all the data before the range-request threshold has been loaded by the browser, the browser will determine which additional byte ranges in the file are necessary to load. It will then issue HTTP Range Requests (HTTP Semantics § range.requests) for at least those ranges.

Note: Browsers are encouraged to coalesce range requests for nearby areas of the file, to minimize the amount of range-request overhead required. Browsers are encouraged to inform these coalescing decisions from network configuration parameters and bandwidth / latency observations.

Note: If the font file has followed all of the organization guidelines above, all information required for laying out content and performing shaping will lie before any of the outline data in the file, and every glyph’s outline will be independent from every other glyph. Therefore, the browser can treat the range-request threshold as being just before outline data begins, and once it has loaded up to that threshold, it can lay out page content. After laying out the page, downloading all the necessary outlines can be done with a collection of independent and parallel range requests. This works particularly well for Chinese, Japanese, and Korean fonts, where 90% or more of the font data is outline data.

Note: Another valid alternative is to treat the entire font as residing on an asynchronous virtual filesystem, and have the browser track which ranges of the font it ended up reading during its normal operation. The browser could then request those regions in range requests.

5.4. Server Behaviors

Servers supporting the range-request IFT method must support range requests (HTTP Semantics § range.requests).

Servers supporting the range-request IFT method should support compression via the Content-Encoding (HTTP Semantics § name-content-encoding) header or the Transfer-Encoding header (HTTP/1.1 § section-6.1), rather than having the font file itself be statically compressed.

Privacy Considerations

Content inference from character set

IFT exposes, to the server hosting a Web font, the set of characters that the browser can already render in a given Web font, and also the set of characters that it cannot render, but wants to (for example, to render a new Web page). For details, see § 4.4.2 Extending the Font Subset.

The purpose of doing so is to allow the server to compute a binary patch to the existing font, adding more characters. Thus, fonts are transferred incrementally, as needed, which greatly reduces> the bytes transferred and the overall network cost..

For some languages, which use a very large character set (Chinese and Japanese are examples) the vast reduction in total bytes transferred means that Web fonts become usable, including on mobile networks, for the first time.

However, for those languages, it is possible that individual requests might be analyzed by a rogue font server to obtain intelligence about the type of content which is being read. It is unclear how feasible this attack is, or the computational complexity required to exploit it, unless the characters being requested are very unusual.

One mitigation, which was originally introduced for reasons of networking efficiency so is likely to be implemented in practice, is to request additional, un-needed characters to dilute the ability to infer what content the user is viewing. Requesting characters which are statistically likely to occur may avoid a subsequent request.

(IFT mandates HTTPS, so no in-the-middle attack is possible; the trust is between the client, and the server hosting the fonts).

Checksums and possible fingerprinting

In the patch subset method 64 bit checksums are generated and transferred between client and server. These are used for error detection, are not persistent across browsing sessions, change frequently in the course of a single browsing session, and thus should not pose a tracking risk.

Also, browsers typically cache resources keyed by the origin domain; thus the checksums and the set of characters the client requires would only be available to that domain and the patch subset server.

Per-origin restriction avoids fingerprinting

As required by [css-fonts-4], Web Fonts must not be accessible in any other Document from the one which either is associated with the @font-face rule or owns the FontFaceSet. Other applications on the device must not be able to access Web Fonts. This avoids information leaking across origins.

Similarly, font palette values must only be available to the documents that reference it. Using an author-defined color palette outside of the documents that reference it would constitute a security leak since the contents of one page would be able to affect other pages, something an attacker could use as an attack vector.

Security Considerations

No Security issues have been raised against this document

Appendix A: Suggested glyph/character ordering

Note: This section describes ordering of characters, not glyph IDs, because the meaning of glyph IDs are not consistent across different fonts. To optimize a particular font according to the ordering listed here, the characters will have to be mapped to glyph IDs inside the font. This approach of mapping characters to glyphs for ordering purposes works particularly well for ideographic languages with large character sets.

Populate suggested character ordering for range-request method

Appendix B: Default Feature Tags and Encoding IDs

Tag Encoded As
aalt 1
abvf default
abvm default
abvs default
afrc 2
akhn default
blwf default
blwm default
blws default
calt default
case 3
ccmp default
cfar default
chws default
cjct default
clig default
cpct 4
cpsp 5
cswh default
curs default
cv01-cv99 6-104
c2pc 105
c2sc 106
dist default
dlig 107
dnom default
dtls default
expt 108
falt 109
fin2 default
fin3 default
fina default
flac default
frac default
fwid 110
half default
haln default
halt 111
hist 112
hkna 113
hlig 114
hngl 115
hojo 116
hwid 117
init default
isol default
ital 118
jalt default
jp78 119
jp83 120
jp90 121
jp04 122
kern default
lfbd 123
liga default
ljmo default
lnum 124
locl default
ltra default
ltrm default
mark default
med2 default
medi default
mgrk 125
mkmk default
mset default
nalt 126
nlck 127
nukt default
numr default
onum 128
opbd 129
ordn 130
ornm 131
palt 132
pcap 133
pkna 134
pnum 135
pref default
pres default
pstf default
psts default
pwid 136
qwid 137
rand default
rclt default
rkrf default
rlig default
rphf default
rtbd 138
rtla default
rtlm default
ruby 139
rvrn default
salt 140
sinf 141
size 142
smcp 143
smpl 144
ss01-ss20 145-164
ssty default
stch default
subs 165
sups 166
swsh 167
titl 168
tjmo default
tnam 169
tnum 170
trad 171
twid 172
unic 173
valt default
vatu default
vchw default
vert default
vhal 174
vjmo default
vkna 175
vkrn default
vpal default
vrt2 default
vrtr default
zero 176

Conformance

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Conformant Algorithms

Requirements phrased in the imperative as part of algorithms (such as "strip any leading space characters" or "return false and abort these steps") are to be interpreted with the meaning of the key word ("must", "should", "may", etc) used in introducing the algorithm.

Conformance requirements phrased as algorithms or specific steps can be implemented in any manner, so long as the end result is equivalent. In particular, the algorithms defined in this specification are intended to be easy to understand and are not intended to be performant. Implementers are encouraged to optimize.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-FONTS-4]
John Daggett; Myles Maxfield; Chris Lilley. CSS Fonts Module Level 4. 21 December 2021. WD. URL: https://www.w3.org/TR/css-fonts-4/
[CSS-FONTS-5]
Myles Maxfield; Chris Lilley. CSS Fonts Module Level 5. 21 December 2021. WD. URL: https://www.w3.org/TR/css-fonts-5/
[FAST-HASH]
ztanml. fast-hash. 22 October 2018. Note. URL: https://github.com/ztanml/fast-hash
[FETCH]
Fetch Standard. 13 December 2021. Living Standard. URL: https://fetch.spec.whatwg.org/
[OPENTYPE]
OpenType specification. URL: http://www.microsoft.com/typography/otspec/default.htm
[PFE-report]
Chris Lilley. Progressive Font Enrichment: Evaluation Report. 15 October 2020. Note. URL: https://www.w3.org/TR/PFE-evaluation/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://datatracker.ietf.org/doc/html/rfc2119
[RFC3284]
D. Korn; et al. The VCDIFF Generic Differencing and Compression Data Format. June 2002. Proposed Standard. URL: https://www.rfc-editor.org/rfc/rfc3284
[RFC7932]
J. Alakuijala; Z. Szabadka. Brotli Compressed Data Format. July 2016. Informational. URL: https://www.rfc-editor.org/rfc/rfc7932
[RFC8081]
C. Lilley. The "font" Top-Level Media Type. February 2017. Proposed Standard. URL: https://www.rfc-editor.org/rfc/rfc8081
[RFC8949]
C. Bormann; P. Hoffman. Concise Binary Object Representation (CBOR). December 2020. Internet Standard. URL: https://www.rfc-editor.org/rfc/rfc8949
[RFC9110]
R. Fielding, Ed.; M. Nottingham, Ed.; J. Reschke, Ed.. HTTP Semantics. June 2022. Internet Standard. URL: https://www.rfc-editor.org/rfc/rfc9110
[RFC9111]
R. Fielding, Ed.; M. Nottingham, Ed.; J. Reschke, Ed.. HTTP Caching. June 2022. Internet Standard. URL: https://www.rfc-editor.org/rfc/rfc9111
[RFC9112]
R. Fielding, Ed.; M. Nottingham, Ed.; J. Reschke, Ed.. HTTP/1.1. June 2022. Internet Standard. URL: https://www.rfc-editor.org/rfc/rfc9112
[Shared-Brotli]
J. Alakuijala; et al. Shared Brotli Compressed Data Format. 27 Jul 2021. Internet Draft. URL: https://datatracker.ietf.org/doc/html/draft-vandevenne-shared-brotli-format-09#section-3.2
[TRUETYPE]
TrueType™ Reference Manual. URL: https://developer.apple.com/fonts/TrueType-Reference-Manual/
[URL]
Anne van Kesteren. URL Standard. Living Standard. URL: https://url.spec.whatwg.org/
[WOFF]
Jonathan Kew; Tal Leming; Erik van Blokland. WOFF File Format 1.0. 13 December 2012. REC. URL: https://www.w3.org/TR/WOFF/
[WOFF2]
Vladimir Levantovsky; Raph Levien. WOFF File Format 2.0. 10 March 2022. REC. URL: https://www.w3.org/TR/WOFF2/

Informative References

[OPENTYPE-VARIATIONS]
OpenType Font Variations Overview. 23 October 2020. Note. URL: https://docs.microsoft.com/en-us/typography/opentype/spec/otvaroverview
[RFC3629]
F. Yergeau. UTF-8, a transformation format of ISO 10646. November 2003. Internet Standard. URL: https://www.rfc-editor.org/rfc/rfc3629
[RFC4648]
S. Josefsson. The Base16, Base32, and Base64 Data Encodings. October 2006. Proposed Standard. URL: https://www.rfc-editor.org/rfc/rfc4648

Issues Index

Using WOFF2 files with range-request mechanism doesn’t seem to be a viable option
Static file compression compatibility with range-request method
Font Collections support
Supporting fonts with composite glyphs via range-request
Populate suggested character ordering for range-request method