[imsc] Support `#font` TTML2 feature (#472)

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

== Support `#font` TTML2 feature ==
Currently the [`#font`](https://www.w3.org/TR/ttml2/#feature-font) TTML2 feature is not listed in IMSC 1.1. This means that its use is _prohibited_. This means that fonts can not be embedded or referenced by explicit URL. 

One of the proposed approaches to w3c/tt-reqs#15 is to use private use areas of the Unicode range and a font that defines glyphs for the code points used. To do that we would almost certainly a need to support `#font` since the only reliable way to make use of private use area code points is to reference the font that defines the glyphs explicitly, or to embed it.

Please view or discuss this issue at https://github.com/w3c/imsc/issues/472 using your GitHub account

Received on Monday, 25 March 2019 11:44:48 UTC