Add a warning about the "pause" operation

Fixes #1294 by giving implementers license to experiment with alternatives to pausing, and pointing out that this can cause subtle event loop behavior changes, but that's OK.

source

@@ -87641,7 +87641,7 @@ dictionary <dfn>PromiseRejectionEventInit</dfn> : <span>EventInit</span> {
 
     <p class="note">This causes one of the following algorithms to continue: the <span>event
     loop</span>'s main set of steps, the <span>perform a microtask checkpoint</span> algorithm, or
-    the <span>execute a compound microtask subtask</span> algorithm to continue.</p>
+    the <span>execute a compound microtask subtask</span> algorithm.</p>
 
    </li>
 
@@ -87677,6 +87677,22 @@ dictionary <dfn>PromiseRejectionEventInit</dfn> : <span>EventInit</span> {
 
   </ol>
 
+  <div class="warning">
+   <p><span data-x="pause">Pausing</span> is highly detrimental to the user experience, especially
+   in scenarios where a single <span>event loop</span> is shared among multiple documents. User
+   agents are encouraged to experiment with alternatives to <span data-x="pause">pausing</span>,
+   such as <span data-x="spin the event loop">spinning the event loop</span> or even simply
+   proceeding without any kind of suspended execution at all, insofar as it is possible to do so
+   while preserving compatibility with existing content. This specification will happily change if
+   a less-drastic alternative is discovered to be web-compatible.</p>
+
+   <p>In the interim, implementers should be aware that the variety of alternatives that user agents
+   might experiment with can change subtle aspects of <span>event loop</span> behavior, including
+   <span data-x="concept-task">task</span> and <span>microtask</span> timing. Implementations should
+   continue experimenting even if doing so causes them to violate the exact semantics implied by the
+   <span>pause</span> operation.</p>
+  </div>
+
   <h5>Generic task sources</h5>
 
   <p>The following <span data-x="task source">task sources</span> are used by a number of mostly