The Timed Text Working Group has published a First Public Working Draft of IMSC Hypothetical Render Model. This specification specifies an Hypothetical Render Model (HRM) that constrains the complexity of an IMSC Document Instance.
The model is not intended as a specification of the processing requirements for implementations. For instance, while the model defines a glyph buffer for the purpose of limiting the number of glyphs displayed at any given point in time, it neither requires the implementation of such a buffer, nor models the sub-pixel character positioning and anti-aliased glyph rendering that can be used to produce text output.
Источник: W3CHome Page Stories Publication Web Design and Applications