Upgrade note to normative text, deprecating special value of 60 secon…

…ds when inappropriate (#158).

spec/ttml2.xml

@@ -17329,12 +17329,13 @@ in the preceding syntax definition.</p>
 <emph>clock-time</emph>, then leading zeroes are used when expressing hours,
 minutes, seconds, and frames less than 10. Minutes are constrained to
 [0&hellip;59], while seconds (including any fractional part) are constrained to the closed
-interval [0,60], where the value 60 applies only to leap seconds.</p>
-<note role="clarification">
-<p>The special seconds value 60 is applicable only when leap seconds are counted; namely, when
-<loc href="#parameter-attribute-timeBase"><att>ttp:timeBase</att></loc> is <code>clock</code>
-and <loc href="#parameter-attribute-clockMode"><att>ttp:clockMode</att></loc> is <code>local</code> or <code>utc</code>.</p>
-</note>
+interval [0,60], where the value 60 applies only to leap seconds.
+
+Except when <loc href="#parameter-attribute-timeBase"><att>ttp:timeBase</att></loc> is <code>clock</code>
+and <loc href="#parameter-attribute-clockMode"><att>ttp:clockMode</att></loc> is <code>local</code> or <code>utc</code>,
+use of the <phrase role="deprecated">special value of 60 seconds to denote a leap second is deprecated</phrase>.
+When performing presentation processing, the appearance of this special value in other time bases or clock modes
+must be interpreted as if the value 59 had been specified.</p>
 <p>If a &lt;time-expression&gt; is expressed in terms of a
 <emph>clock-time</emph> and a <emph>frames</emph> term is specified,
 then the value of this term must be constrained to the interval