IMSC Hypothetical Render Model is a W3C Recommendation

The Timed Text Working Group published IMSC Hypothetical Render Model as a W3C Recommendation. This specification specifies a Hypothetical Render Model (HRM) that constrains the presentation complexity of documents that conform to the Text Profiles specified in any edition of Internet Media Subtitles and Captions ([IMSC]).

The objective of the HRM is to allow subtitle and caption authors and providers to verify that the content they provide does not exceed defined complexity levels, so that playback systems can render the content synchronized with the author-specified display times.

The model is not intended as a specification of the processing requirements for implementations. For instance, while the model defines glyph cache for the purpose of modelling how the number of glyph drawing operations can be reduced, it neither requires the implementation of such a cache, nor models the sub-pixel glyph positioning and anti-aliased glyph rendering that can be used to produce text output.

Furthermore, the model is not intended to constrain readability complexity.