Clean up the event loop and agent correspondence

This closes #5352, by making event loops and agents 1:1. It also adds an
explicit explanation that event loops/agents and implementation threads
are not necessarily 1:1, apart from the restrictions imposed by
JavaScript's forward progress guarantee.

This also closes #4213, as worklet event loops work fine in this
architecture.

This also closes #4674, as browsing contexts changing event loops is
not a problem in this architecture, since they change agents at the same
time. Other problems remain around communication between agents (see
e.g. #3691), and some of that communication is event-loop mediated, but
the specific note discussed there is no longer relevant.

Author: domenic

source

@@ -2824,6 +2824,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x-href="https://tc39.es/ecma262/#sec-candidate-executions">candidate execution</dfn></li>
      <li>The <dfn data-x-href="https://tc39.es/ecma262/#current-realm">current Realm Record</dfn></li>
      <li><dfn data-x-href="https://tc39.es/ecma262/#early-error-rule">early error</dfn></li>
+     <li><dfn data-x-href="https://tc39.es/ecma262/#sec-forward-progress">forward progress</dfn></li>
      <li><dfn data-x-href="https://tc39.es/ecma262/#sec-invariants-of-the-essential-internal-methods">invariants of the essential internal methods</dfn></li>
      <li><dfn data-x-href="https://tc39.es/ecma262/#sec-execution-contexts">JavaScript execution context</dfn></li>
      <li><dfn data-x-href="https://tc39.es/ecma262/#execution-context-stack">JavaScript execution context stack</dfn></li>
@@ -88912,40 +88913,31 @@ dictionary <dfn>PromiseRejectionEventInit</dfn> : <span>EventInit</span> {
 
   </div>
 
-  <h4>Event loops</h4> <!-- <dfn>event loop</dfn> -->
+  <h4>Event loops</h4>
 
   <h5 w-nodev>Definitions</h5>
 
   <p>To coordinate events, user interaction, scripts, rendering, networking, and so forth, user
   agents must use <dfn data-x="event loop" data-lt="event loop" data-export="">event loops</dfn> as
-  described in this section. Each <span>agent</span> has an associated <span>event loop</span>.</p>
-
-  <p class="note">In the future, this standard hopes to define exactly when <span data-x="event
-  loop">event loops</span> can be created or reused.</p>
-
-  <p>A <dfn>window event loop</dfn> is the <span>event loop</span> used by <span
-  data-x="similar-origin window agent">similar-origin window agents</span>. User agents may share an
-  <span>event loop</span> across <span data-x="similar-origin window agent">similar-origin window
-  agents</span>.</p>
-  <!-- Ideally user agents process-isolate similar-origin window agents from each other that are not
-       same site. However, this is not the reality in 2019. -->
-
-  <p class="note">This specification does not currently describe how to handle the complications
-  arising from <span data-x="navigate">navigating</span> between <span data-x="similar-origin window
-  agent">similar-origin window agents</span>. E.g., when a <span>browsing context</span> <span
-  data-x="navigate">navigates</span> from <code data-x="">https://example.com/</code> to <code
-  data-x="">https://shop.example/</code>.</p>
-
-  <p>A <dfn>worker event loop</dfn> is the <span>event loop</span> used by <span data-x="dedicated
-  worker agent">dedicated worker agents</span>, <span data-x="shared worker agent">shared worker
-  agents</span>, and <span data-x="service worker agent">service worker agents</span>. There must be
-  one <span>worker event loop</span> per such <span>agent</span>.</p>
-
-  <p>A <dfn>worklet event loop</dfn> is the <span>event loop</span> used by <span data-x="worklet
-  agent">worklet agents</span>.</p>
-
-  <p class="XXX">As detailed in <a href="https://github.com/whatwg/html/issues/4213">issue #4213</a>
-  the situation for worklets is more complicated.</p>
+  described in this section. Each <span>agent</span> has an associated <span>event loop</span>,
+  which is unique to that agent.</p>
+
+  <p>The <span>event loop</span> of a <span>similar-origin window agent</span> is known as a
+  <dfn>window event loop</dfn>. The <span>event loop</span> of a <span>dedicated worker
+  agent</span>, <span>shared worker agent</span>, or <span>service worker agent</span> is known as a
+  <dfn>worker event loop</dfn>. And the <span>event loop</span> of a <span>worklet agent</span> is
+  known as a <dfn>worklet event loop</dfn>.</p>
+
+  <div class="note">
+   <p><span data-x="event loop">Event loops</span> do not necessarily correspond to implementation
+   threads. For example, multiple <span data-x="window event loop">window event loops</span> could
+   be cooperatively scheduled in a single thread.</p>
+
+   <p>However, because the various worker <span data-x="agent">agents</span> are allocated with
+   [[CanBlock]] set to true, the JavaScript specification does place requirements on them regarding
+   <span>forward progress</span>, which effectively amount to requiring dedicated per-agent threads
+   in those cases.</p>
+  </div>
 
   <div w-nodev>