Tweak "Observing event listeners" section

Addresses @annevk feedback: WICG/EventListenerOptions#20 (comment)

Also exports a definition for the "passive event listener" concept.

dom.bs

@@ -102,6 +102,8 @@ urlPrefix: https://html.spec.whatwg.org/multipage/
             text: effective script origin
             text: origin alias; url: #concept-origin-alias
             text: Unicode serialization of an origin
+        urlPrefix: infrastructure.html
+            text: in parallel
 urlPrefix: https://w3c.github.io/webcomponents/spec/shadow/
 	type: dfn; urlPrefix: #dfn-
 		text: shadow root
@@ -988,8 +990,9 @@ dictionary EventListenerOptions {
   invoked if <a>event</a>'s {{Event/eventPhase}} attribute value is {{Event/AT_TARGET}}.
 
   <dt>{{EventListenerOptions/passive}}
-  <dd>When the <var>passive</var> member is set to true, it indicates that the listener's
-  <b>callback</b> will not cancel the event by invoking {{preventDefault()}}.
+  <dd>When the <var>passive</var> member is set to true, it indicates that the listener
+  is <dfn export for="event listener" id=concept-passive-event-listener>passive</dfn>
+  in that it's <b>callback</b> will not cancel the event by invoking {{preventDefault()}}.
   Invoking {{preventDefault()}} from within such a <b>callback</b> will have no effect.
   <p class="note no-backref">
    Setting <b>passive</b> to true can enable the user agent to perform performance
@@ -1135,21 +1138,21 @@ method must run these steps:
 </ol>
 
 <h3 id=observing-event-listeners>Observing event listeners</h3>
-In general, developers don't expect the presence of an event listener to be
-observable.  The impact of an event listener is determined by it's callback.
-That is, a developer adding a no-op event listener would not expect it to have
+In general, developers do not expect the presence of an <a>event listener</a> to be
+observable.  The impact of an <a>event listener</a> is determined by its <b>callback</b>.
+That is, a developer adding a no-op <a>event listener</a> would not expect it to have
 any side effects.
 
 Unfortunately, some event APIs have been designed such that implementing them
-efficiently requires observing event listeners.  For example, sensor APIs which
+efficiently requires observing <a>event listeners</a>.  For example, sensor APIs which
 enable an underlying device sensor, and touch APIs which can be used to block
-async scrolling.  In some cases this problem can be mitigated by specifying
+asynchronous scrolling.  In some cases this problem can be mitigated by specifying
 the event to be {{Event/cancelable}} only when there is at least one
-non-{{EventListenerOptions/passive}} listener.  For example, non-passive
+non-<a>passive</a> listener.  For example, non-passive
 {{TouchEvent}} listeners must block scrolling, but if all listeners are passive then
-scrolling can be allowed to start asynchronously by making the {{TouchEvent}}
+scrolling can be allowed to start <a>in parallel</a> by making the {{TouchEvent}}
 uncancelable (so that calls to {{Event/preventDefault()}} are ignored).
-New event types should be defined such that they don't need this
+Ideally, any new event types are defined such that they don't need this
 property (use <a href="https://lists.w3.org/Archives/Public/public-script-coord/">public-scrip-coord@w3.org</a>
 for discussion).
 

dom.html

@@ -687,7 +687,8 @@ <h3 class="heading settled" data-level="3.6" id="dictionary-eventlisteneroptions
   invoked if <a data-link-type="dfn" href="#concept-event">event</a>’s <code class="idl"><a data-link-type="idl" href="#dom-event-eventphase">eventPhase</a></code> attribute value is <code class="idl"><a data-link-type="idl" href="#dom-event-at_target">AT_TARGET</a></code>. 
     <dt><code class="idl"><a data-link-type="idl" href="#dom-eventlisteneroptions-passive">passive</a></code> 
     <dd>
-     When the <var>passive</var> member is set to true, it indicates that the listener’s <b>callback</b> will not cancel the event by invoking <code class="idl"><a data-link-type="idl" href="#dom-event-preventdefault">preventDefault()</a></code>.
+     When the <var>passive</var> member is set to true, it indicates that the listener
+  is <dfn data-dfn-for="event listener" data-dfn-type="dfn" data-export="" id="concept-passive-event-listener">passive<a class="self-link" href="#concept-passive-event-listener"></a></dfn> in that it’s <b>callback</b> will not cancel the event by invoking <code class="idl"><a data-link-type="idl" href="#dom-event-preventdefault">preventDefault()</a></code>.
   Invoking <code class="idl"><a data-link-type="idl" href="#dom-event-preventdefault">preventDefault()</a></code> from within such a <b>callback</b> will have no effect. 
      <p class="note no-backref" role="note"> Setting <b>passive</b> to true can enable the user agent to perform performance
    optimizations described in <a href="#observing-event-listeners">§3.8 Observing event listeners</a>. </p>
@@ -767,18 +768,18 @@ <h3 class="heading settled" data-level="3.7" id="interface-eventtarget"><span cl
     <li><a data-link-type="dfn" href="#concept-event-dispatch">Dispatch</a> the <var>event</var> and return the value that returns. 
    </ol>
    <h3 class="heading settled" data-level="3.8" id="observing-event-listeners"><span class="secno">3.8. </span><span class="content">Observing event listeners</span><a class="self-link" href="#observing-event-listeners"></a></h3>
-    In general, developers don’t expect the presence of an event listener to be
-observable.  The impact of an event listener is determined by it’s callback.
-That is, a developer adding a no-op event listener would not expect it to have
+    In general, developers do not expect the presence of an <a data-link-type="dfn" href="#concept-event-listener">event listener</a> to be
+observable.  The impact of an <a data-link-type="dfn" href="#concept-event-listener">event listener</a> is determined by its <b>callback</b>.
+That is, a developer adding a no-op <a data-link-type="dfn" href="#concept-event-listener">event listener</a> would not expect it to have
 any side effects. 
    <p>Unfortunately, some event APIs have been designed such that implementing them
-efficiently requires observing event listeners.  For example, sensor APIs which
+efficiently requires observing <a data-link-type="dfn" href="#concept-event-listener">event listeners</a>.  For example, sensor APIs which
 enable an underlying device sensor, and touch APIs which can be used to block
-async scrolling.  In some cases this problem can be mitigated by specifying
+asynchronous scrolling.  In some cases this problem can be mitigated by specifying
 the event to be <code class="idl"><a data-link-type="idl" href="#dom-event-cancelable">cancelable</a></code> only when there is at least one
-non-<code class="idl"><a data-link-type="idl" href="#dom-eventlisteneroptions-passive">passive</a></code> listener.  For example, non-passive <code class="idl"><a data-link-type="idl" href="http://www.w3.org/TR/touch-events/#touchevent-interface">TouchEvent</a></code> listeners must block scrolling, but if all listeners are passive then
-scrolling can be allowed to start asynchronously by making the <code class="idl"><a data-link-type="idl" href="http://www.w3.org/TR/touch-events/#touchevent-interface">TouchEvent</a></code> uncancelable (so that calls to <code class="idl"><a data-link-type="idl" href="#dom-event-preventdefault">preventDefault()</a></code> are ignored).
-New event types should be defined such that they don’t need this
+non-<a data-link-type="dfn" href="#concept-passive-event-listener">passive</a> listener.  For example, non-passive <code class="idl"><a data-link-type="idl" href="http://www.w3.org/TR/touch-events/#touchevent-interface">TouchEvent</a></code> listeners must block scrolling, but if all listeners are passive then
+scrolling can be allowed to start <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/infrastructure.html#in-parallel">in parallel</a> by making the <code class="idl"><a data-link-type="idl" href="http://www.w3.org/TR/touch-events/#touchevent-interface">TouchEvent</a></code> uncancelable (so that calls to <code class="idl"><a data-link-type="idl" href="#dom-event-preventdefault">preventDefault()</a></code> are ignored).
+Ideally, any new event types are defined such that they don’t need this
 property (use <a href="https://lists.w3.org/Archives/Public/public-script-coord/">public-scrip-coord@w3.org</a> for discussion).</p>
    <h3 class="heading settled" data-level="3.9" id="dispatching-events"><span class="secno">3.9. </span><span class="content">Dispatching events</span><a class="self-link" href="#dispatching-events"></a></h3>
    <p>To <dfn data-dfn-for="Event" data-dfn-type="dfn" data-export="" id="concept-event-dispatch">dispatch<a class="self-link" href="#concept-event-dispatch"></a></dfn> an <a data-link-type="dfn" href="#concept-event">event</a> to a
@@ -5106,7 +5107,12 @@ <h3 class="no-num heading settled" id="index-defined-here"><span class="content"
    <li><a href="#dom-treewalker-parentnode">parentNode()</a><span>, in §6.2</span>
    <li><a href="#partially-contained">partially contained</a><span>, in §5.2</span>
    <li><a href="#concept-tree-participate">participate</a><span>, in §2.1</span>
-   <li><a href="#dom-eventlisteneroptions-passive">passive</a><span>, in §3.6</span>
+   <li>
+    passive
+    <ul>
+     <li><a href="#dom-eventlisteneroptions-passive">dict-member for EventListenerOptions</a><span>, in §3.6</span>
+     <li><a href="#concept-passive-event-listener">dfn for event listener</a><span>, in §3.6</span>
+    </ul>
    <li><a href="#passive-flag">passive flag</a><span>, in §3.2</span>
    <li><a href="#dom-nodeiterator-pointerbeforereferencenode">pointerBeforeReferenceNode</a><span>, in §6.1</span>
    <li><a href="#concept-range-bp-position">position</a><span>, in §5.2</span>