Made smooth transitions a possibility rather than a recommendation (#304

)

spec/ttml1.xml

@@ -7511,7 +7511,7 @@ then the element is associated with the default region;</p></item>
 adjacent synchronic <emph>Document Instances</emph>,
 <emph>DOC<sub><phrase>inter<sub>N</sub></phrase></sub></emph>
 <emph>DOC<sub><phrase>inter<sub>N&minus;1</sub></phrase></sub></emph>,
-then it is preferred that the implementation make the transition between
+then the implementation can apply processing to make the transition between
 presenting the two instances as smooth as possible, e.g., as described
 by <bibref ref="cea608e"/>, &sect; C.3, and <bibref ref="ccreq"/>.</p>
 </note>