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Added support for link rel=compression-dictionary #11620

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e365212
Added support for link rel=compression-dictionary
pmeenan Sep 3, 2025
ac7f722
Fixed CORS mode processing and http link processing
pmeenan Sep 4, 2025
4db4707
Fixed No CORS
pmeenan Sep 4, 2025
dac8718
Fixed link header processing
pmeenan Sep 4, 2025
3d454f8
Fixed algorithm scoping
pmeenan Nov 11, 2025
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Original file line number Diff line number Diff line change
Expand Up @@ -16162,6 +16162,7 @@ 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 @@ -27219,6 +27220,7 @@ 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 @@ -27284,6 +27286,17 @@ 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 @@ -27705,6 +27718,94 @@ 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>
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@domfarolino domfarolino Nov 18, 2025

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I don't think this data-x is required here, if the text content matches it.


<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).


<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 data-x="external resource link">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.

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>Set <var>options</var>'s <span data-x="link options destination">destination</span> to
the empty string.</p></li>
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@domfarolino domfarolino Nov 18, 2025

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This is already the empty string, per https://html.spec.whatwg.org/C#link-options-destination right?


<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-initiator">initiator</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 @@ -155627,6 +155728,9 @@ 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