From W3C Wiki
This is the wiki page for working on the TTML to WebVTT and WebVTT to TTML mapping.
- Style sheet sources and precedence
- Background paint area - block level or glyph only?
- TTML supports writing direction in plain text, WebVTT requires Unicode bidi chars
- Preservation of line breaks and spaces
- In general cases, TTML allows specifying content for which the mapping would require the size of the video
- Guidelines for id mapping
- Empty lines are allowed in TTML and don't have a background.
- TTML allows out-of-order spans in a p
- Background color on region and z-index on region do not map from TTML to WebVTT
- Exact margin value that has to be added to the top and bottom of the video when using line positioning with snapsToLine Flag active (WebVTT) is unclear