Add support for marquee style semantics

[plehegar]

ARIB-TT [1] defines an extension [2] to TTML to allow author to specify marquee semantics. Yoshiharu Dewa (Sony) suggests [3] this feature be supported by TTML.

Before considering adding a marquee style, however, we need to investigate if the new animate element could be used to express equivalent semantics.

[1] http://www.arib.or.jp/english/html/overview/doc/2-STD-B62v1_0-1p2.pdf [2] http://lists.w3.org/Archives/Public/public-tt/2014Nov/0035.html [3] http://lists.w3.org/Archives/Public/public-tt/2014Nov/0037.html

(raised by Glenn Adams on 2014-11-17) From tracker issue http://www.w3.org/AudioVideo/TT/tracker/issues/355

[nigelmegitt]

Without normative region clipping semantics I do not believe that marquee style semantics can be achieved.

For example, to achieve varying inline displacement of text one might imagine that content is placed inside a region where tts:overflow="hidden" and a span precedes the text content, and that some kind of indentation is applied to that span to move the text outside the region, which is varied by animation so that the text is gradually brought inside the region. However two things prevent that now:

1. the semantics of tts:overflow are "should" only so there is no guarantee that the behaviour will be consistent. See also CSS clip property.
2. the closest thing to an indentation is tts:ipd which does not permit negative length specification, so cannot be used to move text out in this way. The CSS equivalent property to achieve this effect would probably be margin.

We should consider if this requirement is needed for TTML2 or if it can be delivered later. The HTML <marquee> element is deprecated in favour of CSS animations, and that work on the CSS marquee specification (https://www.w3.org/TR/css3-marquee/) has been stopped.

[nigelmegitt]

[Meeting 2017-01-13] Agreed to defer adding margin semantics to post-TTML2.

[skynavga]

Marking for ttml.next and closing.