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Overview.html
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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 d765c696b, updated Fri Mar 8 15:58:52 2024 -0800" name="generator">
<link href="https://www.w3.org/TR/IFT/" rel="canonical">
<meta content="023a8d58f4211912688181c9aa0773307cfccf38" 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="2024-06-04">4 June 2024</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> © 2024 <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 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.</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 groups operating under
the <a href="https://www.w3.org/Consortium/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/Consortium/Patent-Policy-20200915/#def-essential">Essential Claim(s)</a> must disclose the information in accordance with <a href="https://www.w3.org/Consortium/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/2023/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="#extend-font-subset"><span class="secno">4.2</span> <span class="content">Incremental Font Extension Algorithm</span></a>
<li><a href="#fully-expanding-a-font"><span class="secno">4.3</span> <span class="content">Fully Expanding a Font</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-woff2"><span class="secno">5.1</span> <span class="content">Incremental Font Transfer and WOFF2</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="#brotli"><span class="secno">6.2</span> <span class="content">Brotli Patch</span></a>
<ol class="toc">
<li><a href="#apply-brotli"><span class="secno">6.2.1</span> <span class="content">Applying Brotli Patches</span></a>
</ol>
<li>
<a href="#per-table-brotli"><span class="secno">6.3</span> <span class="content">Per Table Brotli</span></a>
<ol class="toc">
<li><a href="#apply-per-table-brotli"><span class="secno">6.3.1</span> <span class="content">Applying Per Table Brotli Patches</span></a>
</ol>
<li>
<a href="#glyph-keyed"><span class="secno">6.4</span> <span class="content">Glyph Keyed</span></a>
<ol class="toc">
<li><a href="#apply-glyph-keyed"><span class="secno">6.4.1</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"></span> <span class="content">Privacy Considerations</span></a>
<ol class="toc">
<li><a href="#content-inference-from-character-set"><span class="secno"></span> <span class="content">Content inference from character set</span></a>
<li><a href="#per-origin"><span class="secno"></span> <span class="content">Per-origin restriction avoids fingerprinting</span></a>
</ol>
<li><a href="#sec"><span class="secno"></span> <span class="content">Security Considerations</span></a>
<li><a href="#changes"><span class="secno"></span> <span class="content">Changes</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>
</nav>
<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 codepoints) 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
often the case for Chinese, Japanese, Korean, Emoji, and Icon fonts.</p>
<li data-md>
<p>Only a small amount of text is being rendered. For example a font that is only used for a
headline.</p>
</ul>
<p>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 alter
the rendering of some content <a href="https://www.w3.org/TR/PFE-evaluation/#fail-subset">Progressive Font Enrichment: Evaluation Report § fail-subset</a> if there are layout rules between characters in
different subsets. Incremental font transfer does not suffer from this issue as it can emcompass the
original font and all of it’s layout rules.</p>
<h4 class="heading settled" data-level="1.4.1" id="reduce-requests"><span class="secno">1.4.1. </span><span class="content">Reducing the Number of Network Requests</span><a class="self-link" href="#reduce-requests"></a></h4>
<p>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 augmentation
will typically require at least one round trip time, performance can be negatively impacted if too many requests
are made. Depending on which patch types are available and how much information is provided in the <a href="#font-format-extensions">patch mapping</a>, a client might preemptively request patches for codepoints that are not
currently needed, but expected to be needed in the future. 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 <a href="https://www.w3.org/TR/PFE-evaluation/#codepredict">codepoint prediction</a> scheme and found it improved overall performance.</p>
<h2 class="heading settled" data-level="2" id="opt-in"><span class="secno">2. </span><span class="content">Opt-In Mechanism</span><a class="self-link" href="#opt-in"></a></h2>
<p>Web pages can choose to opt-in to incremental transfer for a font via the use of a CSS font tech
keyword (<a href="https://drafts.csswg.org/css-fonts-4/#font-tech-definitions"><cite>CSS Fonts 4</cite> § 11.1 Font tech</a>) inside the ''@font-face'' block. The keyword <code>incremental</code> is used to indicate the referenced font contains IFT data and should
only be loaded by a user agent which supports incremental font transfer.</p>
<div class="example" id="example-d6868efb">
<a class="self-link" href="#example-d6868efb"></a>
<pre>@font-face {
font-family: "MyCoolWebFont";
src: url("MyCoolWebFont.otf") tech(incremental);
}
</pre>
</div>
<p class="note" role="note"><span class="marker">Note:</span> Each individual <code>@font-face</code> 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.</p>
<p class="note" role="note"><span class="marker">Note:</span> the IFT tech keyword can be used in conjunction with other font tech specifiers to perform
font feature selection. For example a <code>@font-face</code> could include two URIs one with <code>tech(incremental, color-COLRv1)</code> and the other with <code>tech(incremental, color-COLRv0)</code>.</p>
<h3 class="heading settled" data-level="2.1" id="offline-usage"><span class="secno">2.1. </span><span class="content">Offline Usage</span><a class="self-link" href="#offline-usage"></a></h3>
<p>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 fully expand the incremental font
by invoking <a data-link-type="abstract-op" href="#abstract-opdef-fully-expand-a-font-subset" id="ref-for-abstract-opdef-fully-expand-a-font-subset">Fully Expand a Font Subset</a> and replace references to the incremental font with the fully expanded one.</p>
<h2 class="heading settled" data-level="3" id="definitions"><span class="secno">3. </span><span class="content">Definitions</span><a class="self-link" href="#definitions"></a></h2>
<h3 class="heading settled" data-level="3.1" id="font-subset-dfn"><span class="secno">3.1. </span><span class="content">Font Subset</span><a class="self-link" href="#font-subset-dfn"></a></h3>
<p>A <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="font-subset">font subset</dfn> is a modified version of a font file <a data-link-type="biblio" href="#biblio-iso14496-22" title="Information technology — Coding of audio-visual objects — Part 22: Open Font Format">[iso14496-22]</a> that contains only the data
needed to render a subset of:</p>
<ul>
<li data-md>
<p>the codepoints,</p>
<li data-md>
<p><a href="https://docs.microsoft.com/en-us/typography/opentype/spec/featuretags#">layout features</a>,</p>
<li data-md>
<p>and <a href="https://docs.microsoft.com/en-us/typography/opentype/spec/otvaroverview#terminology">design-variation space</a>.</p>
</ul>
<p>supported by the original font. When a subsetted font is used to render text using any combination of the subset
codepoints, <a href="https://docs.microsoft.com/en-us/typography/opentype/spec/featuretags#">layout features</a>, or <a href="https://docs.microsoft.com/en-us/typography/opentype/spec/otvaroverview#terminology">design-variation space</a> it should 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. Design variation spaces
are specified using the user-axis scales (<a href="https://docs.microsoft.com/en-us/typography/opentype/spec/otvaroverview#coordinate-scales-and-normalization">OpenType Specification § otvaroverview#coordinate-scales-and-normalization</a>).</p>
<p>A <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="font-subset-definition">font subset definition</dfn> describes the minimum data (codepoints, layout features,
variation axis space) that a <a data-link-type="dfn" href="#font-subset" id="ref-for-font-subset">font subset</a> should support.</p>
<p class="note" role="note"><span class="marker">Note:</span> For convenience the remainder of this document links to the <a data-link-type="biblio" href="#biblio-open-type" title="OpenType Specification">[open-type]</a> specification which is a copy of <a data-link-type="biblio" href="#biblio-iso14496-22" title="Information technology — Coding of audio-visual objects — Part 22: Open Font Format">[iso14496-22]</a>.</p>
<h3 class="heading settled" data-level="3.2" id="font-patch-definitions"><span class="secno">3.2. </span><span class="content">Font Patch</span><a class="self-link" href="#font-patch-definitions"></a></h3>
<p>A <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="font-patch">font patch</dfn> is a file which encodes changes to be made to an IFT-encoded font. Patches are used to extend
an existing <a data-link-type="dfn" href="#font-subset" id="ref-for-font-subset①">font subset</a> and provide expanded coverage.</p>
<p>A <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="patch-format">patch format</dfn> is a specified encoding of changes to be applied relative to a <a data-link-type="dfn" href="#font-subset" id="ref-for-font-subset②">font subset</a>. A set of
changes encoded according to the format is a <a data-link-type="dfn" href="#font-patch" id="ref-for-font-patch">font patch</a>. Each <a data-link-type="dfn" href="#patch-format" id="ref-for-patch-format">patch format</a> has an associated <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="patch-application-algorithm">patch application algorithm</dfn> which takes a <a data-link-type="dfn" href="#font-subset" id="ref-for-font-subset③">font subset</a> and a <a data-link-type="dfn" href="#font-patch" id="ref-for-font-patch①">font patch</a> encoded in the <a data-link-type="dfn" href="#patch-format" id="ref-for-patch-format①">patch format</a> as input and outputs an extended <a data-link-type="dfn" href="#font-subset" id="ref-for-font-subset④">font subset</a>.</p>
<h3 class="heading settled" data-level="3.3" id="patch-map-dfn"><span class="secno">3.3. </span><span class="content">Patch Map</span><a class="self-link" href="#patch-map-dfn"></a></h3>
<p>A <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="patch-map">patch map</dfn> is an <a href="https://docs.microsoft.com/en-us/typography/opentype/spec/otff#table-directory">open type table</a> which encodes a collection of mappings from <a data-link-type="dfn" href="#font-subset-definition" id="ref-for-font-subset-definition">font subset definitions</a> to URIs which host <a href="#font-patch-formats">patches</a> that extend the <a data-link-type="dfn" href="#incremental-font" id="ref-for-incremental-font①">incremental font</a>. A <a data-link-type="dfn" href="#patch-map" id="ref-for-patch-map">patch map</a> table encodes a list of <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="patch-map-entries">patch map entries</dfn>, where each entry has a key and value.
The key is a <a data-link-type="dfn" href="#font-subset-definition" id="ref-for-font-subset-definition①">font subset definition</a> and the value is a URI, the <a href="#font-patch-formats">§ 6 Font Patch Formats</a> used by the data at the URI, and the <a href="#font-patch-invalidations">compatibility ID</a> of the patch map table. More details of the format of patch maps can be found
in <a href="#font-format-extensions">§ 5 Extensions to the Font Format</a>.</p>
<p><a data-link-type="dfn" href="#patch-map-entries" id="ref-for-patch-map-entries">Patch Map Entry</a> summary:</p>
<table>
<tbody>
<tr>
<th>Key
<th>Value
<tr>
<td>
<ul>
<li data-md>
<p><a data-link-type="dfn" href="#font-subset-definition" id="ref-for-font-subset-definition②">font subset definition</a></p>
</ul>
<td>
<ul>
<li data-md>
<p>Patch URI</p>
<li data-md>
<p><a href="#font-patch-formats">Patch Format</a></p>
<li data-md>
<p><a href="#font-patch-invalidations">compatibility ID</a></p>
</ul>
</table>
<h3 class="heading settled" data-level="3.4" id="data-types"><span class="secno">3.4. </span><span class="content">Explanation of Data Types</span><a class="self-link" href="#data-types"></a></h3>
<p>Encoded data structures in the remainder of this specification are described in terms of the data types defined
in <a href="https://docs.microsoft.com/en-us/typography/opentype/spec/otff#data-types">OpenType Specification § otff#data-types</a>. As with the rest of OpenType, all fields use "big-endian" byte ordering.</p>
<h2 class="heading settled" data-level="4" id="extending-font-subset"><span class="secno">4. </span><span class="content">Extending a Font Subset</span><a class="self-link" href="#extending-font-subset"></a></h2>
<p>This section defines the algorithm that a client uses to extend an <a data-link-type="dfn" href="#incremental-font" id="ref-for-incremental-font②">incremental</a> <a data-link-type="dfn" href="#font-subset" id="ref-for-font-subset⑤">font subset</a> to cover additional
codepoints, layout features and/or design space.</p>
<h3 class="heading settled" data-level="4.1" id="font-patch-invalidations"><span class="secno">4.1. </span><span class="content">Patch Invalidations</span><a class="self-link" href="#font-patch-invalidations"></a></h3>
<p>The application of a patch to an incremental <a data-link-type="dfn" href="#font-subset" id="ref-for-font-subset⑥">font subset</a> may invalidate some or all of the other mapped patches, so that if
one of those were to be applied the result would be a corrupt <a data-link-type="dfn" href="#font-subset" id="ref-for-font-subset⑦">font</a>. Patch validity is tracked by the compatibility ID from the <a href="#patch-map-table">§ 5.2 Patch Map Table</a>. Every patch has a
compatibility ID encoded within it which needs to match the compatibility ID from the <a href="#patch-map-table">§ 5.2 Patch Map Table</a> which lists that patch.
Every patch is categorized into one of three types, depending on how it does or does not invalidate other patches in the map(s):</p>
<ul>
<li data-md>
<p><dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="full-invalidation">Full Invalidation</dfn>: when this patch is applied all other patches currently listed in the <a data-link-type="dfn" href="#font-subset" id="ref-for-font-subset⑧">font subset</a> are invalidated.
The compatibility ID in both the 'IFT ' and 'IFTX' <a href="#patch-map-table">§ 5.2 Patch Map Table</a> will be changed.</p>
<li data-md>
<p><dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="partial-invalidation">Partial Invalidation</dfn>: when this patch is applied all other patches in the same <a href="#patch-map-table">§ 5.2 Patch Map Table</a> will be invalidated.
The compatibility ID of only the <a href="#patch-map-table">§ 5.2 Patch Map Table</a> which contains this patch will be changed.</p>
<li data-md>
<p><dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="no-invalidation">No Invalidation</dfn>: no other patches will be invalidated by the application of this patch. The compatibility ID of the
'IFT ' and 'IFTX' <a href="#patch-map-table">§ 5.2 Patch Map Table</a> will not change.</p>
</ul>
<p>The invalidation behavior of a specific patch is encoded in its format number, which can be found in <a href="#font-patch-formats-summary">§ 6.1 Formats Summary</a>.</p>
<h3 class="heading settled algorithm" data-algorithm="Incremental Font Extension Algorithm" data-level="4.2" id="extend-font-subset"><span class="secno">4.2. </span><span class="content">Incremental Font Extension Algorithm</span><a class="self-link" href="#extend-font-subset"></a></h3>
<p><dfn class="dfn-paneled" data-dfn-type="abstract-op" data-export id="abstract-opdef-extend-an-incremental-font-subset">Extend an Incremental Font Subset</dfn></p>
<p>The inputs to this algorithm are:</p>
<ul>
<li data-md>
<p><var>font subset</var>: an <a data-link-type="dfn" href="#incremental-font" id="ref-for-incremental-font③">incremental</a> <a data-link-type="dfn" href="#font-subset" id="ref-for-font-subset⑨">font subset</a>.</p>
<li data-md>
<p><var>original font subset URI</var>: an <a href="https://www.rfc-editor.org/rfc/rfc3986#section-4.3">absolute URI</a> which identifies the location of the original incremental
font that <var>font subset</var> was derived from.</p>
<li data-md>
<p><var>target subset definition</var>: the <a data-link-type="dfn" href="#font-subset-definition" id="ref-for-font-subset-definition③">font subset definition</a> that the client wants to extend <var>font subset</var> to cover.</p>
</ul>
<p>The algorithm outputs:</p>
<ul>
<li data-md>
<p><var>extended font subset</var>: an extended version of <var>font subset</var>. May or may not be an <a data-link-type="dfn" href="#incremental-font" id="ref-for-incremental-font④">incremental font</a>.</p>
</ul>
<p>The algorithm:</p>
<ol>
<li data-md>
<p>Set <var>extended font subset</var> to <var>font subset</var>.</p>
<li data-md>
<p>Load the 'IFT ' and 'IFTX' (if present) mapping <a href="https://docs.microsoft.com/en-us/typography/opentype/spec/otff#table-directory">tables</a> from <var>extended font subset</var>. Both
tables are formatted as a <a href="#patch-map-table">§ 5.2 Patch Map Table</a>. Check that they are valid according to the requirements in <a href="#patch-map-table">§ 5.2 Patch Map Table</a>. If
either table is not valid, invoke <a data-link-type="abstract-op" href="#abstract-opdef-handle-errors" id="ref-for-abstract-opdef-handle-errors">Handle errors</a>. If <var>extended font subset</var> does not have an 'IFT ' table, then it is
not an <a data-link-type="dfn" href="#incremental-font" id="ref-for-incremental-font⑤">incremental font</a> and cannot be extended, return <var>extended font subset</var>.</p>
<li data-md>
<p>For each of <a href="https://docs.microsoft.com/en-us/typography/opentype/spec/otff#table-directory">tables</a> 'IFT ' and 'IFTX' (if present): convert the table into a list of entries by
invoking <a data-link-type="abstract-op" href="#abstract-opdef-interpret-format-1-patch-map" id="ref-for-abstract-opdef-interpret-format-1-patch-map">Interpret Format 1 Patch Map</a> or <a data-link-type="abstract-op" href="#abstract-opdef-interpret-format-2-patch-map" id="ref-for-abstract-opdef-interpret-format-2-patch-map">Interpret Format 2 Patch Map</a>. Concatenate the returned entry lists into a single list, <var>entry list</var>.</p>
<li data-md>
<p>For each <var>entry</var> in <var>entry list</var> invoke <a data-link-type="abstract-op" href="#abstract-opdef-check-entry-intersection" id="ref-for-abstract-opdef-check-entry-intersection">Check entry intersection</a> with <var>entry</var> and <var>target subset definition</var> as inputs, if it returns false remove <var>entry</var> from <var>entry list</var>.</p>
<li data-md>
<p>Remove any entries in <var>entry list</var> which have a patch URI which was loaded and applied previously during the execution
of this algorithm.</p>
<li data-md>
<p>If <var>entry list</var> is empty, then the extension operation is finished, return <var>extended font subset</var>.</p>