Percentage values in tts:padding do not match CSS

[nigelmegitt]

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.

[nigelmegitt]

[Meeting 2016-11-10] General view that we should match CSS here and that not doing so was an oversight.

[skynavga]

Resolved in d76b1eb.