Adding support for the margin Attribute
TTML did not adopt the margin property, since it was felt that positioning could generally be made explicit using origin and extent, however this overlooks an important feature of the margin attribute; namely the auto value (see [| Computing widths and margins. It is actually extremely difficult to replicate the centering behaviour that the margin attribute allows, which is unfortunate as formatting a centred, but left justified caption is a very common practice in television broadcast.
Furthermore another common practice in television content is to indent the second and subsequent lines in a multi line caption. This is also hard to replicate in TTML 1.0, as it requires explicit positioning of two relative lines, and only regions may be positioned. It can be approximated by using explicit space characters and xml:space preserve, however this is an inaccurate and not a very elegant solution.
This change proposal suggests that we add the margin property into the style properties admissible on block level and inline elements.
- Simplifies conversion from 608 and 708 content.
- Allows greater control of margins within a paragraph.