Add Sec-Purpose header for subresources when prerendering (#382)

Closes #371.

Author: domenic

prefetch.bs

@@ -132,6 +132,7 @@ spec: prerendering-revamped; urlPrefix: prerendering.html
   type: dfn
     text: getting the supported loading modes; url: get-the-supported-loading-modes
     text: uncredentialed-prefetch; for: Supports-Loading-Mode; url: supports-loading-mode-uncredentialed-prefetch
+    text: prerendering navigable; url: prerendering-navigable
     text: prerendering traversable; url: prerendering-traversable
 spec: no-vary-search; urlPrefix: https://httpwg.org/http-extensions/draft-ietf-httpbis-no-vary-search.html
   type: dfn
@@ -888,7 +889,7 @@ The following parameters are defined for the `prefetch` token:
 
 * A parameter whose key is "<dfn for="Sec-Purpose prefetch">`prerender`</dfn>".
 
-  If present with a value other than boolean false (`` `?0` `` in the field value), this parameter indicates that the prefetch request is being made in anticipation of a prerender.
+  If present with a value other than boolean false (`` `?0` `` in the field value), this parameter indicates that the prefetch request is being made in anticipation of a prerender. Note that this value is also set for resources made from within a [=prerendering navigable=], before activation.
 
   <div class="note">
     A future specification might define assign more specific meaning to non-boolean values. For now, they are treated the same as true. Implementations are advised not to emit such values.

prerendering.bs

@@ -97,6 +97,8 @@ spec: nav-speculation; urlPrefix: prefetch.html
       text: prerendering traversable
     for: has a matching prefetch record
       text: checkPrerender
+    for: Sec-Purpose prefetch
+      text: prerender
 spec: RFC8941; urlPrefix: https://www.rfc-editor.org/rfc/rfc8941.html
   type: dfn
     text: structured header; url: #section-1
@@ -679,6 +681,20 @@ Modify <a abstract-op spec="CLEAR-SITE-DATA">clear site data for response</a>'s
         1. [=prefetch record/Cancel and discard=] |prefetchRecord| given |activeDocument|.
 </div>
 
+<h3 id="interaction-with-fetch">Interaction with Fetch</h3>
+
+<div algorithm="HTTP-network-or-cache fetch patch">
+  Modify <a spec=FETCH>HTTP-network-or-cache fetch</a> by adding the following step after the existing step that sets [:Sec-Purpose:]:
+
+  1. Otherwise, if |httpRequest|'s [=request/client=] is an [=environment settings object=] whose [=environment settings object/global object=] is a {{Window}}, and that {{Window}}'s [=Window/navigable=] is a [=prerendering navigable=]:
+    1. Let |purpose| be a [=structured header/List=] containing the [=structured header/Token=] `prefetch`.
+    1. Add a parameter whose key is "<a for="Sec-Purpose prefetch">`prerender`</a>" and whose value is true to the `prefetch` token in |purpose|.
+    1. [=header list/Set a structured field value=] given ([:Sec-Purpose:], |purpose|) in |httpRequest|'s [=request/header list=].
+
+    <p class="note">This includes both subresources within the prerendering navigable, and cases where the prerendering navigable is navigating itself.
+
+</div>
+
 <h2 id="supports-loading-mode">The \`<dfn export http-header><code>Supports-Loading-Mode</code></dfn>\` HTTP response header</h2>
 
 <em>The following section would be added as a sub-section of [[HTML]]'s <a href="https://html.spec.whatwg.org/multipage/browsers.html#browsers">Loading web pages</a> section.</em>