World Wide Web Consortium (W3C) Expands Emmy® Award-Winning Work on Captions and Subtitles for More Accessible Video Content

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W3C's Internet Media Subtitles and Captions 1.0 (IMSC1) is new global standard


Read testimonials from W3C Members — 24 May 2016 — The World Wide Web Consortium (W3C), the global standards organization that develops the foundational technologies for the Web, has published new global guidelines, TTML Profiles for Internet Media Subtitles and Captions 1.0 IMSC1 Recommendation, that will improve accessibility and make it easier and less expensive for distributors of online video content to deliver subtitles and closed captions worldwide.


Picture of a movie clip showing the subtitles


"Previously, content creators and distributors used regional versions and variations of TTML captioning and subtitles so the viewing experience was inconsistent in different parts of the world," said Philippe Le Hegaret, W3C Interaction Domain Leader. "Now, IMSC1 brings expanded new features, better technical quality, and consistency on a global scale."

The harmonization of these technical guidelines for online video captions and subtitles was a collaborative effort among members of the W3C Timed Text Working Group, global experts and external organizations, including the European Broadcasting Union (EBU) and the Society of Motion Picture & Television Engineers (SMPTE).

"IMSC1 is a good example where cooperation between the W3C, the EBU and our members has helped to improve the worldwide subtitling and captioning standards scene," commented EBU's Technology & Innovation Director Simon Fell.  "We worked together to minimize the divergence between existing standards and reduced options to simplify practical uses to enable broader adoption." 


IMSC1 has been adopted by projects across standards organizations worldwide, such as the Interoperable Master Format (SMPTE ST 2067-2) developed by SMPTE, and the ATSC Candidate Standard: Captions and Subtitles (A/343) developed by the Advanced Television Systems Committee (ATSC).

Mark Richer, President of the ATSC, said, "IMSC1 is a key element of A/343, a part of the ATSC 3.0 suite of standards for next-generation television broadcasting - both over the air and hybrid using broadband. We appreciate the efforts of W3C in the development of IMSC1."

Jim Helman, CTO of MovieLabs explains, "Starting with the foundational TTML1 and now with IMSC1 and TTML2, the TTML family of standards gives the industry a common target for worldwide authoring and distribution of subtitles and captions, which greatly facilitates implementation and improves fidelity of experience across borders."

Adding sophisticated features for subtitles and captions

An application of the W3C's Emmy® award-winning Timed Text Markup Language (TTML), TTML Profiles for IMSC1 simplifies authoring and processing of subtitles and captions worldwide by harmonizing popular profiles of TTML, including EBU-TT-D, CFF-TT, SDP-US and SMPTE-TT.  In addition, W3C's TTML Working Group updated the working draft of TTML 2, the second version of Timed Text Markup Language (TTML), which adds features introduced in IMSC1 as well as other improvements, such as additional support for East Asian language typography, stereoscopic presentations, and mapping to HTML and CSS.

IMSC1 applies practical constraints to TTML and introduces a number of popular industry features, including:

  • Forced subtitles. A single IMSC1 document can carry text both for hard-of-hearing viewers and translation of subtitles, allowing the user to choose whether to experience just the translations, or both the translations and the captions for the hard of hearing.
  • Enhanced text alignment and padding. IMSC1 supports captioning styles popular in Europe.
  • Images. IMSC1 supports image-based subtitles in addition to text-based subtitles.
  • Reference fonts and character sets. IMSC1 specifies reference fonts and recommends character sets to improve rendering fidelity.
  • Rendering performance model. The IMSC1 Hypothetical Render Model allows document complexity to be measured at time of authoring, ensuring it can be played downstream.

Compatibility and integration with existing workflows and requirements

Described as a harmonization point for subtitling practices around the world, IMSC1 helps to bring together standards, rather than creating further fragmentation. Based on the W3C eXtensible Markup Language (XML), W3C expects IMSC to evolve with the world captioning and subtitling needs, with future versions of IMSC adopting features from second version of TTML (TTML2), currently in development.

IMSC1 is compatible with common media container formats such as MP4 (ISO/IEC 14496-12), used in consumer formats such as DASH, HLS and CFF, and MXF (SMPTE ST 377-1), and used extensively in professional applications.

IMSC1 integrates with existing workflows, content libraries, and captioning requirements by offering conversion from popular CEA-608/708 (SMPTE RP 2052-10 and RP 2052-11) and EBU STL (EBU Tech 3264) captioning formats.

About the World Wide Web Consortium

The World Wide Web Consortium (W3C) is an international consortium where Member organizations, a full-time staff, and the public work together to develop Web standards. W3C primarily pursues its mission through the creation of Web standards and guidelines designed to ensure long-term growth and stewardship for the Web. Over 400 organizations are Members of the Consortium.

W3C is jointly run by the MIT Computer Science and Artificial Intelligence Laboratory (MIT CSAIL) in the United States, the European Research Consortium for Informatics and Mathematics (ERCIM) headquartered in France, Keio University in Japan and Beihang University in China. W3C has Offices in Australia; the Benelux countries; Brazil; Finland; France; Germany and Austria; Greece; Hungary; India; Italy; Korea; Morocco; Russia; Southern Africa; Spain; Sweden; and the United Kingdom and Ireland. For more information see

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Testimonials from W3C members

British Broadcasting Corporation (BBC)European Broadcasting Union (EBU)Institut für Rundfunktechnik (IRT)Netflix

British Broadcasting Corporation (BBC)

The BBC has been a pioneer in the field of subtitles and captions, providing services to audiences, and technical solutions since the late 1970s. The BBC was instrumental in setting up the W3C Timed Text Working Group and in developing the TTML format, and has followed this up both within the TTWG and in other industry groups including the EBU.

The BBC congratulates the W3C Timed Text Working Group on the publication of the Recommendation for the IMSC 1 profiles of TTML. As a profile of TTML that incorporates EBU-TT-D the IMSC 1 profiles help to bring together standards rather than creating further fragmentation, and meet the BBC's requirements for subtitle delivery and presentation. They can be used for the global delivery of subtitles and captions for broadcast television content.

Nigel Megitt (

European Broadcasting Union (EBU)

EBU's Technology & Innovation director Simon Fell said: "For us standardisation is not so much about creating new standards with many options, but actually the opposite: minimising the divergence between existing standards and reducing options to simplify practical use and enable broad adoption. IMSC is a good example where cooperation between the W3C, the EBU and our Members has helped to improve the worldwide subtitling standards scene."

Frans De Jong (

Institut für Rundfunktechnik (IRT)

Many organisations, like the EBU with EBU-TT-D, use TTML to define a regional profile that fits the requirements of specific access services. With IMSC the W3C now defines a common basis for all the different TTML profiles which is an important step to foster international compatibility of caption formats. Because IMSC makes the EBU defined EBU-TT-D a complete subset and EBU-TT-D is also mandated by HbbTV 2.0, IMSC has the potential to be the common ground globally which enables interoperability for subtitles in a converged world of broadcast and internet.

IRT participates and contributes to the work of the Timed Text Working Group because it believes that W3C is the suitable forum to specify the technical means for global interoperability of subtitle formats. With IMSC the W3C adds important value to subtitles as a multiplatform access service.

Andreas Tai (


At Netflix, we strive to provide the highest quality captions and subtitles at a global scale while preserving the creative intent of the assets. IMSC1 allows us to achieve this goal and represents a significant milestone in the effort to establish a single global standard for subtitles and closed captions. We are fully committed to supporting IMSC1 as the preferred model for subtitle delivery into the Netflix ecosystem. In addition, we are committed to the next steps of TTML2 adoption and enhancing IMSC to fully support advanced Asian layout features. We look forward to advancing this work through our participation in the W3C Timed Text Working Group and through the funding of open-source TTML2 and IMSC implementations.

David Ronca, Director of Encoding Technologies

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