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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