Timed Text Working Group Charter

The mission of the Timed Text Working Group is to develop W3C Recommendations for the representation of timed text in media, including developing and maintaining new versions of the Timed Text Markup Language (TTML) and WebVTT (Web Video Text Tracks) based on implementation experience and interoperability feedback, and the creation of semantic mappings between those languages.

Join the Timed Text Working Group.

Start date 8 April 2023
End date 7 April 2025
Chairs Nigel Megitt (BBC),
Gary Katsevman (W3C Invited Expert)
Team Contacts Atsushi Shimono (0.15 FTE)
Meeting Schedule Teleconferences:Usually once per week.
Face-to-face: Usually no more than twice per year, including once during the W3C's annual Technical Plenary week.

The Working Group will develop W3C Recommendations for the representation of timed text, including the Timed Text Markup Language (TTML) and WebVTT (Web Video Text Tracks).


This group is chartered to develop specifications for the representation of timed text in media.

Timed text means text synchronized with other timed media, e.g. audio and video (2D, 3D, 360º, AR and VR), and includes captions, subtitles, described video (aka video/audio description), karaoke lyrics, etc.

These specifications are intended to be used across the workflow from authoring to end-user presentation, and for both prepared and live applications.


More detailed milestones and updated publication schedules are available on the group publication status page.

Draft state indicates the state of the deliverable at the time of the charter approval. Expected completion indicates when the deliverable is projected to become a Recommendation, or otherwise reach a stable state.

New Normative Specifications

The Working Group intends to develop the following new W3C normative specifications:

Profiles of Timed Text Markup Language 2 (TTML2), and related specifications.

TTML2 can be used to support a variety of applications. Each such application typically uses an associated subset of TTML2 features, known as a profile. Similarly other related specifications may govern such applications, for example by setting performance or complexity constraints.

An example application would be the authoring and exchange of timed text documents that support audio description or dubbing workflows, under development as the Dubbing and Audio description Profiles of TTML2 specification. Such an application might define text associated with media at various times, in one or more languages, as well as any directives for rendering that text into audio.

New Normative Specifications Continuously Under Development

The Working Group intends to develop the following new W3C normative specifications:

WebVTT: The Web Video Text Tracks Format

This specification defines WebVTT, the Web Video Text Tracks format. Its main use is for marking up external text track resources in connection with the HTML <track> element. WebVTT files provide captions or subtitles for video content, and also text video descriptions [MAUR], chapters for content navigation, and more generally any form of metadata that is time-aligned with audio or video content.

Draft state: Candidate Recommendation

Adopted Working Draft: WebVTT: The Web Video Text Tracks Format (4 April 2019)

Produced under Working Group Charter: 2018-2020 charter period of the Timed Text Working Group

Exclusion Draft: https://www.w3.org/TR/2019/CR-webvtt1-20190404/
associated Call for Exclusion on 2019-04-04 ended on 2019-06-03

IMSC Hypothetical Render Model

This specification specifies an Hypothetical Render Model (HRM) that constrains the complexity of an IMSC Document Instance.

Draft state: Working Draft

Adopted Working Draft: IMSC Hypothetical Render Model (6 January 2022)

Produced under Working Group Charter: 2020-2021 charter period of the Timed Text Working group

Exclusion Draft: https://www.w3.org/TR/2021/WD-imsc-hrm-20211109/
associated Call for Exclusion on 2021-11-09 will end on 2022-04-08

Timed Text Markup Language 2 (TTML2) (2nd Edition)

This document specifies the Second Edition of the Timed Text Markup Language (TTML), Version 2, also known as TTML2 (2e), in terms of a vocabulary and semantics thereof.
The Timed Text Markup Language is a content type that represents timed text media for the purpose of interchange among authoring systems. Timed text is textual information that is intrinsically or extrinsically associated with timing information.

Draft state: Candidate Recommendation

Adopted Working Draft: Timed Text Markup Language 2 (TTML2) (2nd Edition) (9 March 2021)

Produced under Working Group Charter: 2020-2021 charter period of the Timed Text Working group

Exclusion Draft: https://www.w3.org/TR/2021/CR-ttml2-20210309/
associated Call for Exclusion on 2021-03-09 ended on 2021-05-08

The Working Group MAY develop additional Recommendation-track specifications.

Existing Normative Specifications

The Working Group MAY update its previously published Normative Specifications.

Other deliverables

The Working Group MAY create or update documents that are not Normative Specifications, including Working Group Notes and other publications such as:

  • Use case and requirement documents
  • Test suite and implementation reports
  • Primer or Best Practice documents to support web developers

The Working Group MAY create registry definitions and registry tables and MAY create such registries to replace or augment existing registries created prior to the formalisation of the Registry track.


Current timelines are documented at TTWG Publications wiki page and will be updated on an occasional basis.

Success Criteria

In order to advance to Proposed Recommendation, each normative feature is expected to meet the requirements set out in §3.1 to demonstrate adequate implementation experience.

Each Recommendation-track Technical Report:

To promote interoperability, all normative changes made to Technical Reports SHOULD have associated tests.

Requirements for advancing the maturity level of features

Within the requirements of the Process each new normative feature will be considered as ready to advance to a higher maturity level if its implementability has been demonstrated.

When considering suitability to advance any feature beyond Candidate Recommendation, at least two independent factors of verification for each normative requirement MUST be demonstrated, which may come, as relevant for that requirement, from any of:

  • Presentation implementation
  • Content-producing implementation
  • Validating implementation

For example, a feature may be advanced beyond Candidate Recommendation if it has been demonstrated to be implementable on the basis of an open source implementation that successfully processes content from an independent implementation.


For all specifications, this Working Group will seek horizontal review for accessibility, internationalization, performance, privacy, and security with the relevant Working and Interest Groups, and with the TAG. Invitation for review must be issued during each major standards-track document transition, including FPWD. The Working Group is encouraged to engage collaboratively with the horizontal review groups throughout development of each specification. The Working Group is advised to seek a review at least 3 months before first entering CR and is encouraged to proactively notify the horizontal review groups when major changes occur in a specification following a review.

Additional technical coordination with the following Groups will be made, per the W3C Process Document:

W3C Groups

Accessible Platform Architectures (APA) Working Group
The mission of the Accessible Platform Architectures Working Group (APA WG) is to ensure W3C specifications provide support for accessibility to people with disabilities.
Audio Description Community Group
This group developed the first Community report for the Audio Description Profile of TTML2.
CSS Working Group
The work of the Working Group coordinates with this group on presentation and layout issues.
Media and Entertainment Interest Group
The Media and Entertainment Interest Group provides a forum for Web and TV technical discussions, review existing work, as well as the relationship between services on the Web and TV services, and identifies requirements and potential solutions to ensure that the Web will function well with TV.
Media Working Group
The work of the Working Group coordinates with this group on client-side media processing.
Web Media Text Tracks Community Group
This group developed the first Community report for the WebVTT format and will continue to explore new features.
Immersive Web Community Group
This group is to help bring high-performance Virtual Reality and Augmented Reality to the open Web.
HTML Working Group
The HTML specification is intended to provide a semantic-level markup language and associated semantic-level scripting APIs for authoring accessible pages on the Web ranging from static documents to dynamic applications. It includes media elements to present video, audio and video text tracks and their associated APIs.
Immersive Captions Community Group
This group researches on captions with immersive media.

Invitation for review SHALL be issued during each major Recommendation-track document transition, including FPWD, and when major changes occur in a specification, and SHOULD be issued at least 3 months before CR.

External Organizations

The Working Group SHOULD, whenever possible, seek interoperability with other timed text formats, API, and applications.

As such, the Working Group SHOULD seek review with the following external organizations.

ISO/IEC JTC-1/SC-29 WG 11 Moving Picture Experts Group (MPEG)
This group is developing standards for coded representation of digital audio and video, including MPEG-4.
Society of Motion Picture and Television Engineers (SMPTE)
This organization was founded to advance theory and development in the motion imaging field. SMPTE produced extensions to TTML1 that are part of SMPTE-TT.
European Broadcasting Union (EBU)
The EBU is an association of national broadcasting organizations, facilitating the exchange of audiovisual content. The EBU has developed EBU-TT, an XML-based format for use in subtitling production and exchange, and EBU-TT-D for use in subtitle distribution, both of which are constrained and extended variants of TTML1.
DVB Project: Technical Module (DVB-TM)
The DVB Project develops specifications for digital television systems, which are turned into standards by international standards bodies such as ETSI or CENELEC. It provides a conduit to other relevant standardisation activities including MPEG for the purpose of meeting the objectives of the DVB Project.
Advanced Television Systems Committee (ATSC)
ATSC develops standards for digital television and depends on external standards for digital closed captioning.
SA WG4 Codec deals with the specifications for speech, audio, video, and multimedia codecs, in both circuit-switched and packet-switched environments.


To be successful, this Working Group is expected to have 6 or more active participants for its duration, including representatives from the key implementors of this specification, and active Editors and Test Leads for each specification. The Chairs, specification Editors, and Test Leads are expected to contribute half of a working day per week towards the (Working|Interest) Group. There is no minimum requirement for other Participants.

The group encourages questions, comments and issues on its public mailing lists and document repositories, as described in Communication.

The group also welcomes non-Members to contribute technical submissions for consideration upon their agreement to the terms of the W3C Patent Policy.

Participants in the group are required (by the W3C Process) to follow the W3C Code of Ethics and Professional Conduct.


Technical discussions for this Working Group are conducted in public: the meeting minutes from teleconference and face-to-face meetings will be archived for public review, and technical discussions and issue tracking will be conducted in a manner that can be both read and written to by the general public. Working Drafts and Editor's Drafts of specifications will be developed in public repositories and may permit direct public contribution requests. The meetings themselves are not open to public participation, however.

Information about the group (including details about deliverables, issues, actions, status, participants, and meetings) will be available from the Timed Text Working Group home page.

Most Timed Text Working Group teleconferences will focus on discussion of particular specifications, and will be conducted on an as-needed basis.

This Working Group primarily conducts its technical work on the public mailing list public-tt@w3.org (archive) or GitHub issues. The public is invited to review, discuss and contribute to this work.

The Working Group MAY use a Member-confidential mailing list member-tt@w3.org (archive) for administrative purposes and, at the discretion of the Chairs and members of the group, for member-only discussions in special cases when a participant requests such a discussion.

Decision Policy

This group will seek to make decisions through consensus and due process, per the W3C Process Document (section 3.3). Typically, an editor or other participant makes an initial proposal, which is then refined in discussion with members of the group and other reviewers, and consensus emerges with little formal voting being required.

However, if a decision is necessary for timely progress and consensus is not achieved after careful consideration of the range of views presented, the Chairs may call for a group vote and record a decision along with any objections.

To afford asynchronous decisions and organizational deliberation, any resolution (including publication decisions) taken in a face-to-face meeting or teleconference will be considered provisional. A call for consensus (CfC) will be issued for all resolutions (for example, via email, GitHub issue or web-based survey), with a response period 10 working days, depending on the chair's evaluation of the group consensus on the issue. If no objections are raised by the end of the response period, the resolution will be considered to have consensus as a resolution of the Working Group.

All decisions made by the group should be considered resolved unless and until new information becomes available or unless reopened at the discretion of the Chairs or the Director.

This charter is written in accordance with the W3C Process Document (Section 3.4, Votes) and includes no voting procedures beyond what the Process Document requires.

Patent Policy

This Working Group operates under the W3C Patent Policy (Version of 15 September 2020). To promote the widest adoption of Web standards, W3C seeks to issue Web specifications that can be implemented, according to this policy, on a Royalty-Free basis. For more information about disclosure obligations for this group, please see the licensing information.


For each deliverable the Working Group MAY choose either the W3C Document license or the W3C Software and Document license.

About this Charter

This charter has been created according to section 5.2 of the Process Document. In the event of a conflict between this document or the provisions of any charter and the W3C Process, the W3C Process shall take precedence.

Charter History

The following table lists details of all changes from the initial charter, per the W3C Process Document (section 5.2.3):

Charter Period Start Date End Date Changes
Initial Charter 2008-08-15 2010-06-30 Restarted the Working Group
Charter Extension 2011-03-31 none
Rechartered 2012-07-25 2014-01-31 Added: TTML 1.0 2nd edition, TTML 1.1
Rechartered 2014-03-27 2016-03-30 Added: WebVTT 1.0, IMSC 1.0
Charter Extension 2016-05-31 none
Rechartered 2016-05-19 2018-03-31 none
Charter Extension 2018-05-31 none
Rechartered 2018-05-30 2020-05-31 none
Rechartered 2019-11-28 2021-12-31 Added: TTML3, TTML Profile for Audio Description
Rechartered 2020-12-15 2021-12-31 New Patent Policy
Charter Extension 2022-03-31 none
Charter Extension 2022-06-30 none
Charter Extension 2022-12-31 none
Charter Extension 2023-03-31 Affiliation of co-chair Gary Katsevman changed from Brightcove to Mux
Charter Extension 2023-04-30 none
Rechartered 2023-04-08 2025-04-07 Added: Dubbing and Audio description Profiles of TTML2 (replacement of TTML Profile for Audio Description), IMSC Hypothetical Render Model
Suspended: TTML3, TTML Profile for Audio Description

Change log

Changes to this document are documented in this section.

  • 2023-08-23: Affiliation of co-chair Gary Katsevman changed from Mux to W3C Invited Expert.