Merge pull request #652 from w3c/issue-0590-default-vs-implicit

Resolve confusion between default and implicit (#590).

spec/ttml2.xml

@@ -17342,9 +17342,9 @@ by <bibref ref="smil3"/>, &sect;5.4.3,
 while taking into account any overriding semantics defined by this specification.</p>
 <note role="clarification">
 <p>As defined by <bibref ref="smil3"/>, &sect;5.4.3,
-if no <att>begin</att> attribute is explicitly specified on a <loc href="#terms-timed-element">timed element</loc>, then a default (implicit) value
-is determined by the nearest time container ancestor; however, in TTML, that ancestor is either
-a parallel time container or a sequential time container, both of which apply a default value of <code>0</code>.</p>
+if no <att>begin</att> attribute is explicitly specified on a <loc href="#terms-timed-element">timed element</loc>,
+then the default value of the attribute is determined by the nearest time container ancestor; however, in TTML, that ancestor is either
+a parallel time container or a sequential time container, both of which apply a default value of <code>0s</code>.</p>
 </note>
 </div3>
 <div3 id="timing-attribute-dur">