diff --git a/source b/source
index 27dd0653fa7..823b468ccdd 100644
--- a/source
+++ b/source
@@ -10236,6 +10236,12 @@ http://software.hixie.ch/utilities/js/live-dom-viewer/?%3C%21DOCTYPE%20HTML%3E%0
   data-x="concept-html-contents">contents</span> that is <dfn>palpable content</dfn> and that does
   not have the <code data-x="attr-hidden">hidden</code> attribute specified.</p>
 
+  <p class="note"><span>Palpable content</span> makes an element non-empty by providing either
+  some descendant non-empty <span data-x="text content">text</span>, or else something users can
+  hear (<code>audio</code> elements) or view (<code>video</code> or <code>img</code> or
+  <code>canvas</code> elements) or otherwise interact with (for example, interactive form
+  controls).</p>
+
   <p>This requirement is not a hard requirement, however, as there are many cases where an element
   can be empty legitimately, for example when it is used as a placeholder which will later be filled
   in by a script, or when the element is part of a template and would on most pages be filled in but