TTML2 Diff Feature Count

During today's call, I mentioned there were 135 new features in TTML2 (as
compared with TTML1). Cyril asked why this was different from the 255 - 114
= 141 count one gets when comparing TTML2 E.1 subsections with TTML1 D.1
subsections.

The reason for the discrepancy is that there are 6 new feature designators
in TTML2 used to label technical features already present in TTML1, but for
which TTML1 did not define feature designator labels. These six new feature
designators are:

#base                   included in TTML1 #profile (*)
#displayAlign-region    included in TTML1 #displayAlign
#displayAlign-relative  included in TTML1 #displayAlign
#extent-auto            included in TTML1 #extent
#extent-length          included in TTML1 #extent
#set                    equivalent to TTML1 #animation

In the case of #base, xml:base is supported in TTML1 only on ttp:features
and ttp:extensions which are covered by TTML1 #profile.

So, we get 135 (new labels, new features) + 6 (new labels, old features) =
141 (new labels).