Move document Accept to constant

fetch.bs

@@ -2368,6 +2368,9 @@ manually. [[!HTML]]
  <li><p>Return <var>potentialDestination</var>.
 </ol>
 
+<p>The <dfn lt="document Accept header value">Document `Accept` header value</dfn> is
+`<code>text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8</code>`.
+
 
 <h3 id=authentication-entries>Authentication entries</h3>
 
@@ -3984,11 +3987,9 @@ the request.
   <ol>
    <li><p>Let <var>value</var> be `<code>*/*</code>`.
 
-   <li><p>Let <var>documentAccept</var> be
-   `<code>text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8</code>`.
 
    <li><p>If <var>request</var>'s <a for=request>initiator</a> is "<code>prefetch</code>", then set
-   <var>value</var> to <var>documentAccept</var>.
+   <var>value</var> to <a lt="document Accept header value">document `Accept` header value</a>.
 
    <li>
     <p>A user agent should set <var>value</var> to the first matching statement, if any, switching
@@ -3999,7 +4000,7 @@ the request.
      <dt>"<code>document</code>"
      <dt>"<code>frame</code>"
      <dt>"<code>iframe</code>"
-     <dd><var>documentAccept</var>
+     <dd><a lt="document Accept header value">document `Accept` header value</a>
 
      <dt>"<code>image</code>"
      <dd>`<code>image/png,image/svg+xml,image/*;q=0.8,*/*;q=0.5</code>`