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Added support for link rel=compression-dictionary #11620

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05bf299
Added support for link rel=compression-dictionary
pmeenan Feb 11, 2026
868e03c
Fixed CORS mode processing and http link processing
pmeenan Feb 11, 2026
25c321f
Fixed No CORS
pmeenan Feb 11, 2026
763cdd8
Fixed link header processing
pmeenan Feb 11, 2026
c5ca086
Fixed algorithm scoping
pmeenan Feb 11, 2026
c9b3eed
Cleanup unnecessary language
pmeenan Feb 11, 2026
64d7121
Set the destination instead of initiator
pmeenan Feb 12, 2026
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Original file line number Diff line number Diff line change
Expand Up @@ -16166,6 +16166,7 @@ interface <dfn interface>HTMLLinkElement</dfn> : <span>HTMLElement</span> {
the processing model, and are supported by the user agent. The possible <span
data-x="concept-supported-tokens">supported tokens</span> are
<code data-x="rel-alternate">alternate</code>,
<code data-x="rel-compression-dictionary">compression-dictionary</code>,
<code data-x="rel-dns-prefetch">dns-prefetch</code>,
<code data-x="rel-expect">expect</code>,
<code data-x="rel-icon">icon</code>,
Expand Down Expand Up @@ -27256,6 +27257,7 @@ document.body.appendChild(wbr);</code></pre>

<p>Keywords that are <dfn>body-ok</dfn> affect whether <code>link</code> elements are
<span>allowed in the body</span>. The <span>body-ok</span> keywords are
<code data-x="rel-compression-dictionary">compression-dictionary</code>,
<code data-x="rel-dns-prefetch">dns-prefetch</code>,
<code data-x="rel-modulepreload">modulepreload</code>,
<code data-x="rel-pingback">pingback</code>,
Expand Down Expand Up @@ -27321,6 +27323,17 @@ document.body.appendChild(wbr);</code></pre>
<td>Gives the permalink for the nearest ancestor section.</td>
</tr>

<tr>
<td><code data-x="rel-compression-dictionary">compression-dictionary</code></td>
<td><span data-x="external resource link">External Resource</span></td>
<td colspan="2"><em>not allowed</em></td>
<td class="yes"> Yes </td>
<td class="yes"> Yes </td>
<td>Specifies that the user agent should preemptively <span data-x="concept-fetch">fetch</span> and cache the
target resource as it is likely to be useful as a compression dictionary for future
<span data-x="concept-fetch">fetches</span>. <ref>RFC9842</ref></td>
</tr>

<tr>
<td><code data-x="rel-dns-prefetch">dns-prefetch</code></td>
<td><span data-x="external resource link">External Resource</span></td>
Expand Down Expand Up @@ -27742,6 +27755,90 @@ document.body.appendChild(wbr);</code></pre>
Link Relation</cite>. <ref>RFC6596</ref></p>


<h5>Link type "<dfn attr-value for="link/rel"><code
data-x="rel-compression-dictionary">compression-dictionary</code></dfn>"</h5>

<p>The <code data-x="rel-compression-dictionary">compression-dictionary</code> keyword may be used
with <code>link</code> elements. This keyword creates an <span data-x="external resource link">external
resource link</span>. This keyword is <span>body-ok</span>.</p>

<p>The <code data-x="rel-compression-dictionary">compression-dictionary</code> keyword indicates
that preemptively <span data-x="concept-fetch">fetching</span> and caching the specified resource
or same-site document is likely to be beneficial, as it is highly likely that the user will
be able to this resource as a compression dictionary for future
<span data-x="concept-fetch">fetches</span>. <ref>RFC9842</ref></p>
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@domfarolino domfarolino Nov 18, 2025

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Is this ref required? We're not referencing any definition from that spec, right? (Maybe I don't understand all the ref rules and Anne can correct me).

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@pmeenan pmeenan Dec 16, 2025

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There are 2 <ref>RFC9842</ref> references in the PR (both around similar language for link) and they are the only references to the compression dictionary RFC. I'm fine with removing both of them but then people would likely have to go through the fetch spec and from there to the RFC to see what compression dictionaries are.

The closest analog I could see in the code is from the img src text when it references the various bitmap formats without explicitly relying on anything in their specifications (around line 30710):

  <p class="note">The requirements above imply that images can be static bitmaps (e.g. PNGs, GIFs,
  JPEGs), single-page vector documents (single-page PDFs, XML files with an SVG document element),
  animated bitmaps (APNGs, animated GIFs), animated vector graphics (XML files with an SVG
  <span>document element</span> that use declarative SMIL animation), and so forth. However, these
  definitions preclude SVG files with script, multipage PDF files, interactive MNG files, HTML
  documents, plain text documents, and the like. <ref>PNG</ref> <ref>GIF</ref> <ref>JPEG</ref>
  <ref>PDF</ref> <ref>XML</ref> <ref>APNG</ref> <ref>SVG</ref> <ref>MNG</ref></p>


<p>There is no default type for resources given by the
<code data-x="rel-compression-dictionary">compression-dictionary</code> keyword.</p>

<p>The appropriate times to <span data-x="fetch and process the linked resource">fetch and
process</span> this type of link are:</p>

<ul>
<li><p>When the <span>external resource link</span> is created on a <code>link</code> element
that is already <span>browsing-context connected</span>.</p></li>

<li><p>When the <span>external resource link</span>'s <code>link</code> element <span>becomes
browsing-context connected</span>.</p></li>

<li><p>When the <code data-x="attr-link-href">href</code> attribute of the <code>link</code>
element of an <span>external resource link</span> that is already <span>browsing-context
connected</span> is changed.</p></li>

<li><p>When the <code data-x="attr-link-crossorigin">crossorigin</code> attribute of the
<code>link</code> element of an <span>external resource link</span> that is already
<span>browsing-context connected</span> is set, changed, or removed.</p></li>
</ul>

<div algorithm>
<p>The <span>fetch and process the linked resource</span> steps for this type of linked resource,
given a <code>link</code> element <var>el</var>, are to <span
data-x="create link options from element">create link options</span> from <var>el</var> and
to <span>load a compression dictionary</span> given the result and <var>el</var>.</p>

<p>The <span>process a link header</span> step for this type of linked resource given a <span
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@domfarolino domfarolino Nov 18, 2025

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My understanding from TPAC is that compression dictionary responses are intended to be processed almost primarily on subresource requests. But this header-processing algorithm does not run in those cases, and in fact we do not have a spec'ed processing model for subresource Link header requests. Is my understanding correct, and do we have tests for this behavior?

We haven't really had a pressing need to spec the Link header processing model on subresource requests, but if that's one of the main and expected ways of engaging with this new feature, and we have tests for it, perhaps it's important enough to invest in.

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@pmeenan pmeenan Dec 16, 2025

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The Link header is independent from the use of the dictionaries for compression and is used to trigger the loading of a stand-alone dictionary (instead of the subresource delta-compression model where a response is a dictionary itself).

The main use case envisioned is for a HTML document to trigger the loading of a dictionary that would be used in future document requests (compressing away the common HTML templates).

That said, I am sure someone would have a use case for triggering a separate dictionary request from subresources and that might be something Chrome currently allows (along with preload and preconnect) though there are ongoing discussions about how those should be handled to avoid tracking concerns.

The dictionary use case for subresource link headers should probably be included in whatever spec work we do to specify the behavior for preconnect and preload.

Right now the only WPT tests that use the Link header are for the document itself.

data-x="link processing options">link processing options</span> <var>options</var> are to
<span>load a compression dictionary</span> given <var>options</var>.</p>

<p>To <dfn>load a compression dictionary</dfn> given a <span>link processing options</span>
<var>options</var> and optional <code>link</code> element <var>el</var>:</p>

<ol>
<li><p>If <var>options</var>'s <span data-x="link options crossorigin">crossorigin</span>
is <span data-x="attr-crossorigin-none">No CORS</span>, set <var>options</var>'s
<span data-x="link options crossorigin">crossorigin</span> to
<span data-x="attr-crossorigin-anonymous">Anonymous</span>.</p></li>

<li><p>Let <var>request</var> be the result of <span data-x="create a link request">creating a
link request</span> given <var>options</var>.</p></li>

<li><p>If <var>request</var> is null, then return.</p></li>

<li><p>Set <var>request</var>'s <span data-x="concept-request-destination">destination</span> to
"<code data-x="">compression-dictionary</code>".</p></li>

<li>
<p>Let <var>processCompressionDictionaryResponse</var> be the following steps given a <span
data-x="concept-response">response</span> <var>response</var> and null, failure, or a
<span>byte sequence</span> <var>bytesOrNull</var>:</p>

<ol>
<li><p>If <var>response</var> is a <span>network error</span> and <var>el</var> is set, <span
data-x="concept-event-fire">fire an event</span> named <code
data-x="event-error">error</code> at <var>el</var>.</p></li>

<li><p>Otherwise, if <var>el</var> is set, <span data-x="concept-event-fire">fire an event</span>
named <code data-x="event-load">load</code> at <var>el</var>.</p></li>
</ol>
</li>

<li><p>The user agent should <span data-x="concept-fetch">fetch</span> <var>request</var>, with
<i data-x="processResponseConsumeBody">processResponseConsumeBody</i> set to
<var>processCompressionDictionaryResponse</var>. User agents may delay the fetching of
<var>request</var> to prioritize other requests that are necessary for the current document.</p></li>
</ol>
</div>

<h5>Link type "<dfn attr-value for="link/rel"><code
data-x="rel-dns-prefetch">dns-prefetch</code></dfn>"</h5>

Expand Down Expand Up @@ -156173,6 +156270,9 @@ INSERT INTERFACES HERE
<dt id="refsRFC8297">[RFC8297]</dt>
<dd><cite><a href="https://httpwg.org/specs/rfc8297.html">An HTTP Status Code for Indicating Hints</a></cite>, K. Oku. IETF.</dd>

<dt id="refsRFC9842">[RFC9842]</dt>
<dd><cite><a href="https://www.rfc-editor.org/rfc/rfc9842">Compression Dictionary Transport</a></cite>, P. Meenan, Y. Weiss. IETF.</dd>

<dt id="refsSCREENORIENTATION">[SCREENORIENTATION]</dt>
<dd><cite><a href="https://w3c.github.io/screen-orientation/">Screen Orientation</a></cite>, M. Cáceres. W3C.</dd>

Expand Down