- OHMS/ the idiosyncratic xml format used by the OHMS project
- SRT/ https://matroska.org/technical/specs/subtitles/srt.html
- WebAnno/ the W3C Web Annotation recommendation http://w3c.github.io/web-annotation/
- WebVTT/ https://w3c.github.io/webvtt/
SRT is the simplest, least flexible, and requires the most presumption regarding format for metadata.
Not covered are YouTube's preferred format: SCC http://www.theneitherworld.com/mcpoodle/SCC_TOOLS/DOCS/SCC_FORMAT.HTML a binary format
Or iTunes': TTML https://www.w3.org/TR/ttml1/ (iTT subset documented elsewhere)