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]). https://www.w3.org/news/2024/imsc-hypothetical-render-model-is-a-w3c-recommendation/