TTML
TTML (Timed Text Markup Language) is an XML-based W3C standard for storing subtitles and captions with precise timing, styling and positioning. It supports rich formatting such as colors, fonts and placement on screen. Profiles such as IMSC and SMPTE-TT are widely used for broadcast, DASH streaming and professional subtitle delivery.
Why it matters
Studios and broadcasters often require TTML because it keeps caption styling and placement exact. For OTT services, accepting TTML speeds up licensing deals and meets strict accessibility requirements. Getting it wrong can mean captions that cover faces or lose formatting.
How it works
- Captions are written as an XML document with a timeline of paragraphs.
- Each paragraph has begin and end times plus the caption text.
- Style and region elements set fonts, colors and where captions appear on screen.
- Players or packagers read the XML, often in a profile such as IMSC, and render the captions.
TTML vs WebVTT: which should I use?
Use TTML, usually in its IMSC profile, when partners or broadcast workflows require it, and WebVTT for most web and HLS players. TTML offers more precise styling and positioning but is more complex. Many services accept TTML from vendors and convert it to WebVTT for web delivery.
What is IMSC?
IMSC (Internet Media Subtitles and Captions) is a W3C profile of TTML that limits the full standard to features practical for streaming. It comes in text and image variants. IMSC is used in DASH, CMAF and HLS, where Apple accepts IMSC1 subtitles in fragmented MP4.
Where it's used
OTT services and broadcasters ask subtitle vendors for TTML because it keeps caption colors, fonts and placement consistent on every device. Most creator apps use simpler SRT or WebVTT files instead.
By the numbers
- TTML2 (Timed Text Markup Language 2) became a W3C Recommendation on November 8, 2018. (Timed Text Markup Language 2 (W3C))
