Accept header for prefetch

Complements whatwg/html#8111.

Co-authored-by: Anne van Kesteren <annevk@annevk.nl>

Author: noamr

fetch.bs

@@ -1321,6 +1321,9 @@ downloads. This format of range header value can be set using <a>add a range hea
 <a>implementation-defined</a> <a for=/>header value</a> for the `<code>User-Agent</code>`
 <a for=/>header</a>.
 
+<p>The <dfn>document `<code>Accept</code>` header value</dfn> is
+`<code>text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8</code>`.
+
 
 <h4 id=statuses>Statuses</h4>
 
@@ -4274,16 +4277,19 @@ the response. [[!HTTP-CACHING]]
   <ol>
    <li><p>Let <var>value</var> be `<code>*/*</code>`.
 
+   <li><p>If <var>request</var>'s <a for=request>initiator</a> is "<code>prefetch</code>", then set
+   <var>value</var> to the <a>document `<code>Accept</code>` header value</a>.
+
    <li>
-    <p>A user agent should set <var>value</var> to the first matching statement, if any, switching
-    on <var>request</var>'s <a for=request>destination</a>:
+    <p>Otherwise, the user agent should set <var>value</var> to the first matching statement, if
+    any, switching on <var>request</var>'s <a for=request>destination</a>:
     <!-- https://github.com/whatwg/fetch/issues/43#issuecomment-97909717 -->
 
     <dl class=switch>
      <dt>"<code>document</code>"
      <dt>"<code>frame</code>"
      <dt>"<code>iframe</code>"
-     <dd>`<code>text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8</code>`
+     <dd>the <a>document `<code>Accept</code>` header value</a>
 
      <dt>"<code>image</code>"
      <dd>`<code>image/png,image/svg+xml,image/*;q=0.8,*/*;q=0.5</code>`