Deliver TTML/IMSC subtitles for OTT streaming platforms

Steps

1. Author subtitles in IMSC 1.1 (a W3C-defined profile of TTML), which is the profile required by major OTT platforms including Netflix and Amazon — declare the profile with 'ttp:profile' in the TTML root element
2. Structure the XML with a 'tt' root element using the TTML namespace 'http://www.w3.org/ns/ttml', include 'head' with 'styling' and 'layout' sections, and place timed 'p' elements inside a 'div' in the 'body'
3. Use absolute SMPTE or media time expressions for 'begin' and 'end' attributes on each 'p' element (e.g., begin='00:01:12.500' end='00:01:15.000') — wall-clock expressions are not reliable in VOD delivery
4. Embed or reference styling: font-family, font-size (as a percentage of the safe area), color, and background-color should be defined in the 'styling' section and referenced via 'style' attributes on 'span' or 'p' elements
5. For DASH delivery, reference the TTML file as an 'AdaptationSet' with mimeType 'application/ttml+xml' and the appropriate 'lang' attribute in the MPD manifest; for HLS, WebVTT is preferred and TTML is less commonly used in HLS
6. Validate the output file against the IMSC validator (available as an open-source npm package 'imsc') and check rendering with a reference IMSC renderer before delivery to platforms