diff --git a/spec/ttml1.xml b/spec/ttml1.xml
index fe12d393..c7864dfe 100644
--- a/spec/ttml1.xml
+++ b/spec/ttml1.xml
@@ -3320,6 +3320,11 @@ above if possible.</p>
 <p>The semantics of the above four cited XSL-FO properties are defined by
 by <bibref ref="xsl11"/>, &sect; 7.17.3, 7.16.7, 7.16.12, and 7.16.8, respectively.</p>
 </note>
+<note role="clarification">
+<p>No presentation semantics are specified for the TAB (U+0009) character. Furthermore, the TAB (U+0009) character 
+can generate a glyph area. As a result, the use of the TAB (U+0009) character 
+in <code>#PCDATA</code> content within <code>p</code> and <code>span</code> elements is not recommended.</p>
+</note>
 </div3>
 </div2>
 </div1>