Clarify composedPath() method steps with an assert

Closes #1343.

Author: annevk

dom.bs

@@ -523,6 +523,8 @@ was initialized to. When an <a>event</a> is created the attribute must be initia
 
  <li><p>Let <var>currentTarget</var> be <a>this</a>'s {{Event/currentTarget}} attribute value.
 
+ <li><p><a for=/>Assert</a>: <var>currentTarget</var> is an {{EventTarget}} object.
+
  <li><p><a for=list>Append</a> <var>currentTarget</var> to <var>composedPath</var>.
 
  <li><p>Let <var>currentTargetIndex</var> be 0.