--- a/ttml10/spec/ttaf1-dfxp.xml	Sat May 25 18:49:07 2013 -0600
+++ b/ttml10/spec/ttaf1-dfxp.xml	Sat May 25 20:29:25 2013 -0600
@@ -4973,6 +4973,7 @@
 to the after edge.
 If four <loc href="#style-value-length">&lt;length&gt;</loc> specifications are provided, then they apply to before, end,
 after, and start edges, respectively.</p>
+<p>The <loc href="#style-value-length">&lt;length&gt;</loc> value(s) used to express padding must be non-negative.</p>
 <p>If a specified value of this attribute is not supported,
 then a presentation processor must interpret the attribute as if the
 value <code>0px</code> were specified.</p>