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Overview.html
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<!doctype html><html lang="en">
<head>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
<meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
<title>Incremental Font Transfer</title>
<meta content="ED" name="w3c-status">
<link href="https://www.w3.org/StyleSheets/TR/2021/W3C-ED" rel="stylesheet">
<meta content="Bikeshed version ac5ea272d, updated Fri Dec 6 15:45:15 2024 -0800" name="generator">
<link href="https://www.w3.org/TR/IFT/" rel="canonical">
<meta content="6be69980674a9ff91ef3b7d88752b687809580df" name="revision">
<meta content="dark light" name="color-scheme">
<link href="https://www.w3.org/StyleSheets/TR/2021/dark.css" media="(prefers-color-scheme: dark)" rel="stylesheet" type="text/css">
<style>
.conform:hover {background: #31668f; color: white}
.conform:target {padding: 2px; border: 2px solid #AAA; background: #31668f; color: white }
table {
width: 100%;
}
table, tr {
border: 1px solid #aaa;
border-collapse: collapse;
}
th, td {
padding: 0.5rem;
}
</style>
<style>/* Boilerplate: style-autolinks */
.css.css, .property.property, .descriptor.descriptor {
color: var(--a-normal-text);
font-size: inherit;
font-family: inherit;
}
.css::before, .property::before, .descriptor::before {
content: "‘";
}
.css::after, .property::after, .descriptor::after {
content: "’";
}
.property, .descriptor {
/* Don't wrap property and descriptor names */
white-space: nowrap;
}
.type { /* CSS value <type> */
font-style: italic;
}
pre .property::before, pre .property::after {
content: "";
}
[data-link-type="property"]::before,
[data-link-type="propdesc"]::before,
[data-link-type="descriptor"]::before,
[data-link-type="value"]::before,
[data-link-type="function"]::before,
[data-link-type="at-rule"]::before,
[data-link-type="selector"]::before,
[data-link-type="maybe"]::before {
content: "‘";
}
[data-link-type="property"]::after,
[data-link-type="propdesc"]::after,
[data-link-type="descriptor"]::after,
[data-link-type="value"]::after,
[data-link-type="function"]::after,
[data-link-type="at-rule"]::after,
[data-link-type="selector"]::after,
[data-link-type="maybe"]::after {
content: "’";
}
[data-link-type].production::before,
[data-link-type].production::after,
.prod [data-link-type]::before,
.prod [data-link-type]::after {
content: "";
}
[data-link-type=element],
[data-link-type=element-attr] {
font-family: Menlo, Consolas, "DejaVu Sans Mono", monospace;
font-size: .9em;
}
[data-link-type=element]::before { content: "<" }
[data-link-type=element]::after { content: ">" }
[data-link-type=biblio] {
white-space: pre;
}
@media (prefers-color-scheme: dark) {
:root {
--selflink-text: black;
--selflink-bg: silver;
--selflink-hover-text: white;
}
}
</style>
<style>/* Boilerplate: style-colors */
/* Any --*-text not paired with a --*-bg is assumed to have a transparent bg */
:root {
color-scheme: light dark;
--text: black;
--bg: white;
--unofficial-watermark: url(https://www.w3.org/StyleSheets/TR/2016/logos/UD-watermark);
--logo-bg: #1a5e9a;
--logo-active-bg: #c00;
--logo-text: white;
--tocnav-normal-text: #707070;
--tocnav-normal-bg: var(--bg);
--tocnav-hover-text: var(--tocnav-normal-text);
--tocnav-hover-bg: #f8f8f8;
--tocnav-active-text: #c00;
--tocnav-active-bg: var(--tocnav-normal-bg);
--tocsidebar-text: var(--text);
--tocsidebar-bg: #f7f8f9;
--tocsidebar-shadow: rgba(0,0,0,.1);
--tocsidebar-heading-text: hsla(203,20%,40%,.7);
--toclink-text: var(--text);
--toclink-underline: #3980b5;
--toclink-visited-text: var(--toclink-text);
--toclink-visited-underline: #054572;
--heading-text: #005a9c;
--hr-text: var(--text);
--algo-border: #def;
--del-text: red;
--del-bg: transparent;
--ins-text: #080;
--ins-bg: transparent;
--a-normal-text: #034575;
--a-normal-underline: #bbb;
--a-visited-text: var(--a-normal-text);
--a-visited-underline: #707070;
--a-hover-bg: rgba(75%, 75%, 75%, .25);
--a-active-text: #c00;
--a-active-underline: #c00;
--blockquote-border: silver;
--blockquote-bg: transparent;
--blockquote-text: currentcolor;
--issue-border: #e05252;
--issue-bg: #fbe9e9;
--issue-text: var(--text);
--issueheading-text: #831616;
--example-border: #e0cb52;
--example-bg: #fcfaee;
--example-text: var(--text);
--exampleheading-text: #574b0f;
--note-border: #52e052;
--note-bg: #e9fbe9;
--note-text: var(--text);
--noteheading-text: hsl(120, 70%, 30%);
--notesummary-underline: silver;
--assertion-border: #aaa;
--assertion-bg: #eee;
--assertion-text: black;
--advisement-border: orange;
--advisement-bg: #fec;
--advisement-text: var(--text);
--advisementheading-text: #b35f00;
--warning-border: red;
--warning-bg: hsla(40,100%,50%,0.95);
--warning-text: var(--text);
--amendment-border: #330099;
--amendment-bg: #F5F0FF;
--amendment-text: var(--text);
--amendmentheading-text: #220066;
--def-border: #8ccbf2;
--def-bg: #def;
--def-text: var(--text);
--defrow-border: #bbd7e9;
--datacell-border: silver;
--indexinfo-text: #707070;
--indextable-hover-text: black;
--indextable-hover-bg: #f7f8f9;
--outdatedspec-bg: rgba(0, 0, 0, .5);
--outdatedspec-text: black;
--outdated-bg: maroon;
--outdated-text: white;
--outdated-shadow: red;
--editedrec-bg: darkorange;
}
@media (prefers-color-scheme: dark) {
:root {
--text: #ddd;
--bg: black;
--unofficial-watermark: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='400' height='400'%3E%3Cg fill='%23100808' transform='translate(200 200) rotate(-45) translate(-200 -200)' stroke='%23100808' stroke-width='3'%3E%3Ctext x='50%25' y='220' style='font: bold 70px sans-serif; text-anchor: middle; letter-spacing: 6px;'%3EUNOFFICIAL%3C/text%3E%3Ctext x='50%25' y='305' style='font: bold 70px sans-serif; text-anchor: middle; letter-spacing: 6px;'%3EDRAFT%3C/text%3E%3C/g%3E%3C/svg%3E");
--logo-bg: #1a5e9a;
--logo-active-bg: #c00;
--logo-text: white;
--tocnav-normal-text: #999;
--tocnav-normal-bg: var(--bg);
--tocnav-hover-text: var(--tocnav-normal-text);
--tocnav-hover-bg: #080808;
--tocnav-active-text: #f44;
--tocnav-active-bg: var(--tocnav-normal-bg);
--tocsidebar-text: var(--text);
--tocsidebar-bg: #080808;
--tocsidebar-shadow: rgba(255,255,255,.1);
--tocsidebar-heading-text: hsla(203,20%,40%,.7);
--toclink-text: var(--text);
--toclink-underline: #6af;
--toclink-visited-text: var(--toclink-text);
--toclink-visited-underline: #054572;
--heading-text: #8af;
--hr-text: var(--text);
--algo-border: #456;
--del-text: #f44;
--del-bg: transparent;
--ins-text: #4a4;
--ins-bg: transparent;
--a-normal-text: #6af;
--a-normal-underline: #555;
--a-visited-text: var(--a-normal-text);
--a-visited-underline: var(--a-normal-underline);
--a-hover-bg: rgba(25%, 25%, 25%, .2);
--a-active-text: #f44;
--a-active-underline: var(--a-active-text);
--borderedblock-bg: rgba(255, 255, 255, .05);
--blockquote-border: silver;
--blockquote-bg: var(--borderedblock-bg);
--blockquote-text: currentcolor;
--issue-border: #e05252;
--issue-bg: var(--borderedblock-bg);
--issue-text: var(--text);
--issueheading-text: hsl(0deg, 70%, 70%);
--example-border: hsl(50deg, 90%, 60%);
--example-bg: var(--borderedblock-bg);
--example-text: var(--text);
--exampleheading-text: hsl(50deg, 70%, 70%);
--note-border: hsl(120deg, 100%, 35%);
--note-bg: var(--borderedblock-bg);
--note-text: var(--text);
--noteheading-text: hsl(120, 70%, 70%);
--notesummary-underline: silver;
--assertion-border: #444;
--assertion-bg: var(--borderedblock-bg);
--assertion-text: var(--text);
--advisement-border: orange;
--advisement-bg: #222218;
--advisement-text: var(--text);
--advisementheading-text: #f84;
--warning-border: red;
--warning-bg: hsla(40,100%,20%,0.95);
--warning-text: var(--text);
--amendment-border: #330099;
--amendment-bg: #080010;
--amendment-text: var(--text);
--amendmentheading-text: #cc00ff;
--def-border: #8ccbf2;
--def-bg: #080818;
--def-text: var(--text);
--defrow-border: #136;
--datacell-border: silver;
--indexinfo-text: #aaa;
--indextable-hover-text: var(--text);
--indextable-hover-bg: #181818;
--outdatedspec-bg: rgba(255, 255, 255, .5);
--outdatedspec-text: black;
--outdated-bg: maroon;
--outdated-text: white;
--outdated-shadow: red;
--editedrec-bg: darkorange;
}
/* In case a transparent-bg image doesn't expect to be on a dark bg,
which is quite common in practice... */
img { background: white; }
}
</style>
<style>/* Boilerplate: style-counters */
body {
counter-reset: example figure issue;
}
.issue {
counter-increment: issue;
}
.issue:not(.no-marker)::before {
content: "Issue " counter(issue);
}
.example {
counter-increment: example;
}
.example:not(.no-marker)::before {
content: "Example " counter(example);
}
.invalid.example:not(.no-marker)::before,
.illegal.example:not(.no-marker)::before {
content: "Invalid Example" counter(example);
}
figcaption {
counter-increment: figure;
}
figcaption:not(.no-marker)::before {
content: "Figure " counter(figure) " ";
}
</style>
<style>/* Boilerplate: style-dfn-panel */
:root {
--dfnpanel-bg: #ddd;
--dfnpanel-text: var(--text);
--dfnpanel-target-bg: #ffc;
--dfnpanel-target-outline: orange;
}
@media (prefers-color-scheme: dark) {
:root {
--dfnpanel-bg: #222;
--dfnpanel-text: var(--text);
--dfnpanel-target-bg: #333;
--dfnpanel-target-outline: silver;
}
}
.dfn-panel {
position: absolute;
z-index: 35;
width: 20em;
width: 300px;
height: auto;
max-height: 500px;
overflow: auto;
padding: 0.5em 0.75em;
font: small Helvetica Neue, sans-serif, Droid Sans Fallback;
background: var(--dfnpanel-bg);
color: var(--dfnpanel-text);
border: outset 0.2em;
white-space: normal; /* in case it's moved into a pre */
}
.dfn-panel:not(.on) { display: none; }
.dfn-panel * { margin: 0; padding: 0; text-indent: 0; }
.dfn-panel > b { display: block; }
.dfn-panel a { color: var(--dfnpanel-text); }
.dfn-panel a:not(:hover) { text-decoration: none !important; border-bottom: none !important; }
.dfn-panel a:focus {
outline: 5px auto Highlight;
outline: 5px auto -webkit-focus-ring-color;
}
.dfn-panel > b + b { margin-top: 0.25em; }
.dfn-panel ul { padding: 0 0 0 1em; list-style: none; }
.dfn-panel li a {
max-width: calc(300px - 1.5em - 1em);
overflow: hidden;
text-overflow: ellipsis;
}
.dfn-panel.activated {
display: inline-block;
position: fixed;
left: 8px;
bottom: 2em;
margin: 0 auto;
max-width: calc(100vw - 1.5em - .4em - .5em);
max-height: 30vh;
transition: left 1s ease-out, bottom 1s ease-out;
}
.dfn-panel .link-item:hover {
text-decoration: underline;
}
.dfn-panel .link-item .copy-icon {
opacity: 0;
}
.dfn-panel .link-item:hover .copy-icon,
.dfn-panel .link-item .copy-icon:focus {
opacity: 1;
}
.dfn-panel .copy-icon {
display: inline-block;
margin-right: 0.5em;
width: 0.85em;
height: 1em;
border-radius: 3px;
background-color: #ccc;
cursor: pointer;
}
.dfn-panel .copy-icon .icon {
width: 100%;
height: 100%;
background-color: #fff;
display: flex;
justify-content: center;
align-items: center;
position: relative;
}
.dfn-panel .copy-icon .icon::before {
content: "";
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
border: 1px solid black;
background-color: #ccc;
opacity: 0.25;
transform: translate(3px, -3px);
}
.dfn-panel .copy-icon:active .icon::before {
opacity: 1;
}
.dfn-paneled[role="button"] { cursor: help; }
.highlighted {
animation: target-fade 3s;
}
@keyframes target-fade {
from {
background-color: var(--dfnpanel-target-bg);
outline: 5px solid var(--dfnpanel-target-outline);
}
to {
color: var(--a-normal-text);
background-color: transparent;
outline: transparent;
}
}
</style>
<style>/* Boilerplate: style-issues */
a[href].issue-return {
float: right;
float: inline-end;
color: var(--issueheading-text);
font-weight: bold;
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<body class="h-entry">
<div class="head">
<p data-fill-with="logo"><a class="logo" href="https://www.w3.org/"> <img alt="W3C" height="48" src="https://www.w3.org/StyleSheets/TR/2021/logos/W3C" width="72"> </a> </p>
<h1 class="p-name no-ref" id="title">Incremental Font Transfer</h1>
<p id="w3c-state"><a href="https://www.w3.org/standards/types/#ED">Editor’s Draft</a>, <time class="dt-updated" datetime="2025-01-18">18 January 2025</time></p>
<div data-fill-with="spec-metadata">
<dl>
<dt>This version:
<dd><a class="u-url" href="https://w3c.github.io/IFT/Overview.html">https://w3c.github.io/IFT/Overview.html</a>
<dt>Latest published version:
<dd><a href="https://www.w3.org/TR/IFT/">https://www.w3.org/TR/IFT/</a>
<dt>Feedback:
<dd><span><a href="mailto:[email protected]?subject=%5BIFT%5D%20YOUR%20TOPIC%20HERE">[email protected]</a> with subject line “<kbd>[IFT] <i data-lt>… message topic …</i></kbd>” (<a href="https://lists.w3.org/Archives/Public/public-webfonts-wg/" rel="discussion">archives</a>)</span>
<dd><a href="https://github.com/w3c/PFE/issues/">GitHub</a>
<dt class="editor">Editors:
<dd class="editor p-author h-card vcard" data-editor-id="1438"><a class="p-name fn u-url url" href="https://svgees.us/">Chris Lilley</a> (<span class="p-org org">W3C</span>)
<dd class="editor p-author h-card vcard" data-editor-id="73905"><a class="p-name fn u-email email" href="mailto:[email protected]">Garret Rieger</a> (<span class="p-org org">Google Inc.</span>)
<dd class="editor p-author h-card vcard" data-editor-id="137857"><a class="p-name fn u-email email" href="mailto:[email protected]">Skef Iterum</a> (<span class="p-org org">Adobe Inc.</span>)
</dl>
</div>
<div data-fill-with="warning"></div>
<p class="copyright" data-fill-with="copyright"><a href="https://www.w3.org/policies/#copyright">Copyright</a> © 2025 <a href="https://www.w3.org/">World Wide Web Consortium</a>. <abbr title="World Wide Web Consortium">W3C</abbr><sup>®</sup> <a href="https://www.w3.org/policies/#Legal_Disclaimer">liability</a>, <a href="https://www.w3.org/policies/#W3C_Trademarks">trademark</a> and <a href="https://www.w3.org/copyright/software-license/" rel="license" title="W3C Software and Document License">permissive document license</a> rules apply. </p>
<hr title="Separator for header">
</div>
<div class="p-summary" data-fill-with="abstract">
<h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
<p>This specification defines a method to incrementally transfer a font
from server to client. The client loads only the portion(s) of the font that
they actually need, significantly reducing data transfer. Multiple incremental
additions to the same font are possible, such as a user agent updating a font
as a user browses multiple pages. Incremental transfer improves on
unicode-range by avoiding damage to layout (kerning, ligatures, etc) rules,
meaning it can efficiently support fine grained increments to latin and
to complex scripts like Indic or Arabic.</p>
</div>
<h2 class="no-num no-toc no-ref heading settled" id="sotd"><span class="content">Status of this document</span></h2>
<div data-fill-with="status">
<p> This is a public copy of the editors’ draft.
It is provided for discussion only and may change at any moment.
Its publication here does not imply endorsement of its contents by W3C.
Don’t cite this document other than as work in progress. </p>
<p> If you wish to make comments regarding this document, please <a href="https://github.com/w3c/PFE/issues/new">file an issue on the specification repository</a>. </p>
<p> This document was produced by the <a href="https://www.w3.org/groups/wg/webfonts">Web Fonts Working Group</a> </p>
<p> This document was produced by a group operating under
the <a href="https://www.w3.org/policies/patent-policy/20200915/">W3C Patent Policy</a>.
W3C maintains a <a href="https://www.w3.org/groups/wg/webfonts/ipr" rel="disclosure">public list of any patent disclosures</a> 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 <a href="https://www.w3.org/policies/patent-policy/20200915/#def-essential">Essential Claim(s)</a> must disclose the information in accordance with <a href="https://www.w3.org/policies/patent-policy/20200915/#sec-Disclosure">section 6 of the <abbr title="World Wide Web Consortium">W3C</abbr> Patent Policy</a>. </p>
<p> This document is governed by the <a href="https://www.w3.org/policies/process/20231103/" id="w3c_process_revision">03 November 2023 W3C Process Document</a>. </p>
<p></p>
</div>
<div data-fill-with="at-risk"></div>
<nav data-fill-with="table-of-contents" id="toc">
<h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
<ol class="toc" role="directory">
<li>
<a href="#intro"><span class="secno">1</span> <span class="content">Introduction</span></a>
<ol class="toc">
<li><a href="#evaluation-report"><span class="secno">1.1</span> <span class="content">Technical Motivation: Evaluation Report</span></a>
<li><a href="#overview"><span class="secno">1.2</span> <span class="content">Overview</span></a>
<li><a href="#making-incremental-fonts"><span class="secno">1.3</span> <span class="content">Creating an Incremental Font</span></a>
<li>
<a href="#performance-considerations"><span class="secno">1.4</span> <span class="content">Performance Considerations and the use of Incremental Font Transfer</span></a>
<ol class="toc">
<li><a href="#reduce-requests"><span class="secno">1.4.1</span> <span class="content">Reducing the Number of Network Requests</span></a>
</ol>
</ol>
<li>
<a href="#opt-in"><span class="secno">2</span> <span class="content">Opt-In Mechanism</span></a>
<ol class="toc">
<li><a href="#offline-usage"><span class="secno">2.1</span> <span class="content">Offline Usage</span></a>
</ol>
<li>
<a href="#definitions"><span class="secno">3</span> <span class="content">Definitions</span></a>
<ol class="toc">
<li><a href="#font-subset-dfn"><span class="secno">3.1</span> <span class="content">Font Subset</span></a>
<li><a href="#font-patch-definitions"><span class="secno">3.2</span> <span class="content">Font Patch</span></a>
<li><a href="#patch-map-dfn"><span class="secno">3.3</span> <span class="content">Patch Map</span></a>
<li><a href="#data-types"><span class="secno">3.4</span> <span class="content">Explanation of Data Types</span></a>
</ol>
<li>
<a href="#extending-font-subset"><span class="secno">4</span> <span class="content">Extending a Font Subset</span></a>
<ol class="toc">
<li><a href="#font-patch-invalidations"><span class="secno">4.1</span> <span class="content">Patch Invalidations</span></a>
<li><a href="#default-layout-features"><span class="secno">4.2</span> <span class="content">Default Layout Features</span></a>
<li><a href="#extend-font-subset"><span class="secno">4.3</span> <span class="content">Incremental Font Extension Algorithm</span></a>
<li><a href="#invalidating-patch-selection"><span class="secno">4.4</span> <span class="content">Selecting Invalidating Patches</span></a>
<li><a href="#target-subset-definitions"><span class="secno">4.5</span> <span class="content">Target Subset Definition</span></a>
<li><a href="#ift-font-coverage"><span class="secno">4.6</span> <span class="content">Determining what Content a Font can Render</span></a>
<li><a href="#fully-expanding-a-font"><span class="secno">4.7</span> <span class="content">Fully Expanding a Font</span></a>
<li><a href="#caching-incremental-fonts"><span class="secno">4.8</span> <span class="content">Caching Extended Incremental Fonts</span></a>
</ol>
<li>
<a href="#font-format-extensions"><span class="secno">5</span> <span class="content">Extensions to the Font Format</span></a>
<ol class="toc">
<li><a href="#ift-and-compression"><span class="secno">5.1</span> <span class="content">Incremental Font Transfer and Font Compression Formats</span></a>
<li>
<a href="#patch-map-table"><span class="secno">5.2</span> <span class="content">Patch Map Table</span></a>
<ol class="toc">
<li>
<a href="#patch-map-format-1"><span class="secno">5.2.1</span> <span class="content">Patch Map Table: Format 1</span></a>
<ol class="toc">
<li><a href="#interpreting-patch-map-format-1"><span class="secno">5.2.1.1</span> <span class="content">Interpreting Format 1</span></a>
<li><a href="#remove-entries-format-1"><span class="secno">5.2.1.2</span> <span class="content">Remove Entries from Format 1</span></a>
</ol>
<li>
<a href="#patch-map-format-2"><span class="secno">5.2.2</span> <span class="content">Patch Map Table: Format 2</span></a>
<ol class="toc">
<li><a href="#interpreting-patch-map-format-2"><span class="secno">5.2.2.1</span> <span class="content">Interpreting Format 2</span></a>
<li><a href="#remove-entries-format-2"><span class="secno">5.2.2.2</span> <span class="content">Remove Entries from Format 2</span></a>
<li><a href="#sparse-bit-set-decoding"><span class="secno">5.2.2.3</span> <span class="content">Sparse Bit Set</span></a>
</ol>
<li><a href="#uri-templates"><span class="secno">5.2.3</span> <span class="content">URI Templates</span></a>
</ol>
</ol>
<li>
<a href="#font-patch-formats"><span class="secno">6</span> <span class="content">Font Patch Formats</span></a>
<ol class="toc">
<li><a href="#font-patch-formats-summary"><span class="secno">6.1</span> <span class="content">Formats Summary</span></a>
<li>
<a href="#table-keyed"><span class="secno">6.2</span> <span class="content">Table Keyed</span></a>
<ol class="toc">
<li><a href="#apply-table-keyed"><span class="secno">6.2.1</span> <span class="content">Applying Table Keyed Patches</span></a>
</ol>
<li>
<a href="#glyph-keyed"><span class="secno">6.3</span> <span class="content">Glyph Keyed</span></a>
<ol class="toc">
<li><a href="#apply-glyph-keyed"><span class="secno">6.3.1</span> <span class="content">Applying Glyph Keyed Patches</span></a>
</ol>
</ol>
<li>
<a href="#encoding"><span class="secno">7</span> <span class="content">Encoding</span></a>
<ol class="toc">
<li><a href="#encoding-considerations"><span class="secno">7.1</span> <span class="content">Encoding Considerations</span></a>
</ol>
<li>
<a href="#priv"><span class="secno">8</span> <span class="content">Privacy Considerations</span></a>
<ol class="toc">
<li><a href="#content-inference-from-character-set"><span class="secno">8.1</span> <span class="content">Content inference from character set</span></a>
<li><a href="#per-origin"><span class="secno">8.2</span> <span class="content">Per-origin restriction avoids fingerprinting</span></a>
</ol>
<li><a href="#sec"><span class="secno">9</span> <span class="content">Security Considerations</span></a>
<li><a href="#changes"><span class="secno">10</span> <span class="content">Changes</span></a>
<li><a href="#feature-tag-list"><span class="secno"></span> <span class="content"> Appendix A: Default Feature Tags</span></a>
<li><a href="#extension-example"><span class="secno"></span> <span class="content"> Appendix B: Extension Algorithm Example Execution</span></a>
<li>
<a href="#w3c-conformance"><span class="secno"></span> <span class="content">Conformance</span></a>
<ol class="toc">
<li><a href="#w3c-conventions"><span class="secno"></span> <span class="content">Document conventions</span></a>
<li><a href="#w3c-conformant-algorithms"><span class="secno"></span> <span class="content">Conformant Algorithms</span></a>
</ol>
<li>
<a href="#index"><span class="secno"></span> <span class="content">Index</span></a>
<ol class="toc">
<li><a href="#index-defined-here"><span class="secno"></span> <span class="content">Terms defined by this specification</span></a>
</ol>
<li>
<a href="#references"><span class="secno"></span> <span class="content">References</span></a>
<ol class="toc">
<li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
<li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
</ol>
</ol>
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<main>
<h2 class="heading settled" data-level="1" id="intro"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="#intro"></a></h2>
<p><em>This section is not normative.</em></p>
<p>Incremental Font Transfer (IFT) is a technology 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. Unlike traditional font subsetting approaches Incremental Font Transfer
retains the encoding of layout rules between segments (<a href="https://www.w3.org/TR/PFE-evaluation/#fail-subset">Progressive Font Enrichment: Evaluation Report § fail-subset</a>).</p>
<p>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 <a data-link-type="biblio" href="#biblio-woff2" title="WOFF File Format 2.0">[WOFF2]</a>, fonts for CJK languages can be too large to be practical.</p>
<h3 class="heading settled" data-level="1.1" id="evaluation-report"><span class="secno">1.1. </span><span class="content">Technical Motivation: Evaluation Report</span><a class="self-link" href="#evaluation-report"></a></h3>
<p>See the Progressive Font Enrichment: Evaluation Report <a data-link-type="biblio" href="#biblio-pfe-report" title="Progressive Font Enrichment: Evaluation Report">[PFE-report]</a> for the investigation which led
to this specification.</p>
<h3 class="heading settled" data-level="1.2" id="overview"><span class="secno">1.2. </span><span class="content">Overview</span><a class="self-link" href="#overview"></a></h3>
<p>An <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="incremental-font">incremental font</dfn> is a regular <a data-link-type="biblio" href="#biblio-open-type" title="OpenType Specification">OpenType</a> font that is reformatted to include incremental functionality,
partly in virtue of two additional <a href="https://docs.microsoft.com/en-us/typography/opentype/spec/otff#table-directory">tables</a>. Using these new tables the font can be augmented
(eg. to cover more code points) by loading and applying patches to it.</p>
<p>The IFT technology has four main pieces:</p>
<ul>
<li data-md>
<p><a href="#extending-font-subset">§ 4 Extending a Font Subset</a>: provides the algorithm that is used by a client to select and apply patches.</p>
<li data-md>
<p><a href="#font-format-extensions">§ 5 Extensions to the Font Format</a>: defines the new tables which contain a list of patches that are available to be applied to a font.</p>
<li data-md>
<p><a href="#font-patch-formats">§ 6 Font Patch Formats</a>: defines three different types of patches that can be used. Two are "generic" binary patches, one is
specific to the font’s format for storing glyph data.</p>
<li data-md>
<p><a href="#encoding">§ 7 Encoding</a>: creates the font and associated patches that form an incremental font.</p>
</ul>
<p>At a high level an <a data-link-type="dfn" href="#incremental-font" id="ref-for-incremental-font">incremental font</a> is used like this:</p>
<ol>
<li data-md>
<p>The client downloads an initial font file, which contains some initial subset of data from the full version of the font along with <a href="#font-format-extensions">embedded data</a> describing the set of <a href="#font-patch-formats">patches</a> which can be used to extend
the font.</p>
<li data-md>
<p>Based on the content to be rendered, the client <a href="#extending-font-subset">selects, downloads, and applies</a> patches to extend
the font to cover additional characters, layout features, and/or variation space. This step is repeated each time there is new
content.</p>
</ol>
<h3 class="heading settled" data-level="1.3" id="making-incremental-fonts"><span class="secno">1.3. </span><span class="content">Creating an Incremental Font</span><a class="self-link" href="#making-incremental-fonts"></a></h3>
<p>It is expected that the most common way to produce an incremental font will be to convert an existing font to use the incremental
encoding defined in this specification. At a high level converting an existing font to be incremental will look like this:</p>
<ol>
<li data-md>
<p>Choose the content of the initial <a href="#font-subset-dfn">subset</a>, this will be included in the initial font file the client loads
and usually consists of any data from the original font that is expected to always be needed.</p>
<li data-md>
<p>Choose the patch type or types. Different patch types have different qualities, so different use cases can call for different
patch types, or in some cases a mix of two types.</p>
<li data-md>
<p>Choose a segmentation of the font. Individual segments will be added to the base subset by the client using patches. Choosing an
appropriate segmentation is one of the most important parts of producing an efficient encoding.</p>
<li data-md>
<p>Based on these choices, generate a set of patches, where each patch adds the data for a particular segment relative to either
the initial font file or a previous patch.</p>
<li data-md>
<p>Generate the initial font file including the initial subset and a <a href="#font-format-extensions">patch mapping</a>. This mapping
lists all of the available patch files, the url they reside at, and information on what data the patch will add to the font.</p>
</ol>
<p class="note" role="note"><span class="marker">Note:</span> this is a highly simplified description of creating an incremental font, a more in-depth discussion of generating an encoding and
requirements on the encoding can be found in the <a href="#encoding">§ 7 Encoding</a> section.</p>
<h3 class="heading settled" data-level="1.4" id="performance-considerations"><span class="secno">1.4. </span><span class="content">Performance Considerations and the use of Incremental Font Transfer</span><a class="self-link" href="#performance-considerations"></a></h3>
<p>Using incremental transfer may not always be beneficial, depending on the characteristics of the font,
the network, and the content being rendered. This section provides non-normative guidance to help decide
when incremental transfer should be utilized.</p>
<p>It is common for an incremental font to trigger the loading of multiple patches in parallel. So to maximize performance, when
serving an incremental font it is recommended that an HTTP server which is capable of multiplexing (such as <a data-link-type="biblio" href="#biblio-rfc9113" title="HTTP/2">[rfc9113]</a> or <a data-link-type="biblio" href="#biblio-rfc9114" title="HTTP/3">[rfc9114]</a>)
is used.</p>
<p>Incrementally loading a font has a fundamental performance trade off 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.</p>
<p>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
(<a href="https://www.w3.org/TR/PFE-evaluation/#langtype">Progressive Font Enrichment: Evaluation Report § langtype</a>). See it’s conclusions <a href="https://www.w3.org/TR/PFE-evaluation/#conclusions">Progressive Font Enrichment: Evaluation Report § conclusions</a> for a discussion of the
anticipated performance of incremental font transfer across the language categories.</p>
<p>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:</p>
<ul>
<li data-md>
<p>If the font contains support for several languages but a user is expected to only render content
in a subset of those languages.</p>
<li data-md>
<p>If the content being rendered uses a small subset of the total characters in a font. This is