diff --git a/dom.bs b/dom.bs
index a3e8b3a5..0d3e4b4a 100644
--- a/dom.bs
+++ b/dom.bs
@@ -8625,6 +8625,10 @@ and {{Range/getBoundingClientRect()}} methods are defined in other specification
 to filter and traverse <a>node</a>
 <a>trees</a>.
 
+Each {{NodeIterator}} and {{TreeWalker}} object has an associated <dfn export
+id=concept-traversal-active for=traversal>active flag</dfn> to avoid
+recursive invocations.  It is initially unset.
+
 Each {{NodeIterator}} and {{TreeWalker}} object also
 has an associated <dfn export id=concept-traversal-root for=traversal>root</dfn>
 <a>node</a>,
@@ -8635,6 +8639,7 @@ To <dfn export id=concept-node-filter for=Node>filter</dfn> <var>node</var> run
 these steps:
 
 <ol>
+ <li>If the <a for=traversal>active flag</a> is set, throw an {{InvalidStateError}}.
  <li>Let <var>n</var> be <var>node</var>'s
  {{Node/nodeType}} attribute value minus 1.
  <li>If the <var>n</var><sup>th</sup> bit (where 0 is the least
@@ -8644,9 +8649,11 @@ these steps:
  <!-- !((1 << (node.nodeType - 1)) & whatToShow) -->
  <li>If <a for=traversal>filter</a> is null,
  return {{NodeFilter/FILTER_ACCEPT}}.
+ <li>Set the <a for=traversal>active flag</a>.
  <li>Let <var>result</var> be the return value of <a>call a user object's operation</a> with
  <a for=traversal>filter</a>, "<code>acceptNode</code>", and a list of arguments consisting of
  <var>node</var>.
+ <li>Unset the <a for=traversal>active flag</a>.
  <!-- no need to pass callback this value; it's undefined which becomes the global object -->
  <li>Return <var>result</var>.
 </ol>
@@ -8680,7 +8687,7 @@ Each {{NodeIterator}} object has an associated
 <a>node</a>.
 
 <p class="note no-backref">As mentioned earlier, {{NodeIterator}} objects have an
-associated
+associated <a for=traversal>active flag</a>,
 <a for=traversal>root</a> <a>node</a>,
 <a for=traversal>whatToShow</a> bitmask, and
 <a for=traversal>filter</a> callback as well.