define a "have a listener for" concept on EventTarget

dom.bs

@@ -995,6 +995,13 @@ and a <dfn export for=EventTarget>legacy-canceled-activation behavior</dfn> algo
 <p class="note no-backref">These algorithms only exist for checkbox and radio <{input}> elements and
 are not to be used for anything else. [[!HTML]]
 
+<p>An {{EventTarget}} object <var>target</var> is said to <dfn export>have a listener for</dfn>
+<var>type</var> if <var>target</var>'s <a>event listener list</a> <a for=list>contains</a> an
+<a>event listener</a> whose <a for="event listener">type</a> is <var>type</var>.
+
+<p class="note no-backref">The presence, or absence, of event listeners
+can not be relied on to design new APIs, see [[#observing-event-listeners]].
+
 <dl class=domintro>
  <dt><code><var>target</var> = new <a constructor for=EventTarget lt=EventTarget()>EventTarget</a>();</code>
  <dd><p>Creates a new {{EventTarget}} object, which can be used by developers to <a>dispatch</a> and