Add a render-blocking boolean flag to request

fetch.bs

@@ -1869,6 +1869,14 @@ otherwise, it is unset.
 <a for=request>done flag</a>, and <a for=request>timing allow failed flag</a> are used as
 bookkeeping details by the <a for=/>fetch</a> algorithm.
 
+<p>A <a for=/>request</a> has an associated boolean
+<dfn export for=request id=concept-request-render-blocking>render-blocking</dfn>.
+Unless stated otherwise, it is false.
+
+<p class=note>This flag is for exclusive use by HTML's render-blocking mechanism. [[!HTML]]
+<!-- This can also be used to override the priority hints.
+     See https://github.com/WICG/priority-hints/issues/69 -->
+
 <hr>
 
 <p>A <dfn export>subresource request</dfn> is a <a for=/>request</a>
@@ -6782,6 +6790,7 @@ interface Request {
   readonly attribute boolean keepalive;
   readonly attribute boolean isReloadNavigation;
   readonly attribute boolean isHistoryNavigation;
+  readonly attribute boolean isRenderBlocking;
   readonly attribute AbortSignal signal;
 
   [NewObject] Request clone();
@@ -6957,6 +6966,11 @@ object), initially null.
  <dd>Returns a boolean indicating whether or not <var>request</var> is for a history
  navigation (a.k.a. back-foward navigation).
 
+ <dt><code><var>request</var> . <a attribute for=Request>isRenderBlocking</a></code>
+ <dd>Returns a boolean indicating whether or not <var>request</var> is for an element that is
+ render-blocking.
+ <!-- How to I link to the definition of render-blocking in the HTML spec? -->
+
  <dt><code><var>request</var> . <a attribute for=Request>signal</a></code>
  <dd>Returns the signal associated with <var>request</var>, which is an
  {{AbortSignal}} object indicating whether or not <var>request</var> has been aborted, and its abort
@@ -7112,6 +7126,9 @@ constructor steps are:
    <dt><a for=request>history-navigation flag</a>
    <dd><var>request</var>'s <a for=request>history-navigation flag</a>.
 
+   <dt><a for=request>render-blocking</a>
+   <dd><var>request</var>'s <a for=request>render-blocking</a>.
+
    <dt><a for=request>URL list</a>
    <dd>A <a for=list>clone</a> of <var>request</var>'s <a for=request>URL list</a>.
   </dl>