[ttml2] Percentage values in `tts:padding` do not match CSS

nigelmegitt has just created a new issue for 
https://github.com/w3c/ttml2:

== Percentage values in `tts:padding` do not match CSS ==
TTML2 retains the wording from TTML1 regarding percentage units for 
the `tts:padding` attribute:

> relative to width and height of region

However TTML1 only specifies padding for `region`. Now that TTML2 also
 specifies padding for the elements `body`, `div`, `p` and `span`, 
this text no longer matches CSS.

https://www.w3.org/TR/CSS2/box.html#propdef-padding specifies that 
percentages:

> refer to width of containing block

TTML2 should follow CSS here and make percentages be relative to the 
area generated for the element on which the padding attribute is 
specified.

Please view or discuss this issue at 
https://github.com/w3c/ttml2/issues/222 using your GitHub account