Clarify percentage units in padding specification (#205)

spec/ttml1.xml

@@ -5078,6 +5078,13 @@ before edge, the second applies to the start and end edges, and the third applie
 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>
+<note role="clarification">
+<p>Percentage values are relative to the dimension of the region to which they apply. For example, if the before and after edges correspond 
+to the top and bottom edges of the region, as is the case for Latin text where <att>tts:writingMode</att> is equal to <code>"lrtb"</code>, 
+then percentage values that apply to either of the two edges are relative to the height of the region. Conversely, if the before and after 
+edges correspond to the right and left edges of the region, as is the case for Japanese text where <att>tts:writingMode</att> is equal to 
+<code>"tbrl"</code>, then percentage values that apply to either of the two edges are relative to the width of the region.</p>
+</note>
 <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 computed value of the property associated with this attribute is not supported,
 then a presentation processor must use the closest supported value.</p>