define a "has listeners for" algorithm on EventTarget

dom.bs

@@ -995,6 +995,18 @@ 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>Each {{EventTarget}} object also has an associated <dfn export>has listeners for</dfn> algorithm,
+which takes a <a for="event listener">type</a> <var>type</var>, and returns a <var>boolean</var>.
+
+<p>To determine if a {{EventTarget}} object <var>eventTarget</var>
+<dfn export>has listeners for</dfn> a given <var>type</var>, run these steps:
+
+<ol>
+ <li><p>If <var>eventTarget</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>, return true.
+ <li><p>Otherwise, return false.
+</ol>
+
 <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