The Timed Text Working Group has published the following specifications as W3C Recommendations:

• Timed Text Markup Language 1 (TTML1) (Third Edition)
• Timed Text Markup Language 2 (TTML2)
• TTML Profiles for Internet Media Subtitles and Captions 1.1

The Timed Text Markup Language is a content type that represents timed text media for the purpose of interchange among authoring systems. TTML Profiles are intended to be used across subtitle and caption delivery applications worldwide, thereby simplifying interoperability, consistent rendering and conversion to other subtitling and captioning formats.

Original source: https://www.w3.org/blog/news/archives/7381