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Timed Text Working Group


The Timed Text Working Group (TTWG) develops W3C Recommendations for online media captioning by developing and maintaining new versions and profiles of TTML (Timed Text Markup Language) and WebVTT (Web Video Text Tracks) based on implementation experience and interoperability feedback, and the creation of semantic mappings between those languages. The group also develops usage notes and manages registries to support implementors.

This is the Wiki for the activities associated with the updating of the Timed Text publications.

Key documents:

The TTWG operates under the Latest Operative W3C Process for all documents currently on the Recommendation Track.

TTML Pages


The group's publications are listed in detail on the Publications page.

Some important Documents

Document Status Publication Date
TTML1 (Third Edition) Recommendation 08 November 2018
TTML2 Recommendation 08 November 2018
TTML2 (2nd Edition) Candidate Rec Snapshot 09 March 2021
WebVTT Candidate Rec 04 April 2019
IMSC1.2 Recommendation 04 August 2020
IMSC1.1 Recommendation 08 November 2018
IMSC1.0.1 Recommendation 24 April 2018
DAPT Editor's Draft n/a
Mapping Between TTML and WebVTT Editor's Draft (Note) n/a
TTML Media Type Definition and Profile Registry Working Group Note 19 November 2020.
TTML Role Registry wiki n/a.
Using the ITU BT.2100 PQ EOTF with the PNG Format Editor's Draft (Note) n/a

The TTML specification allows users to define and use roles for elements, in order to manage this across disparate organisations (without resorting to using complex namespaced prefixes) a set of roles is recorded in the TTML role registry page.

Test Suites

  • IMSC Test Suite. Consists of IMSC documents and corresponding exemplar renders as PNG images and provides coverage for IMSC 1.0.1, IMSC 1.1 and IMSC 1.2 Text and Image profile documents.

Implementations and Specifications

See Timed Text Efforts and Specifications for a high-level list of TTML and WebVTT implementations. TTML1-3ED implementation report

See the IMSC1 implementation page for implementations and test cases for the IMSC1 specification. and IMSC1.0.1 Comments tracker.

TTWG Working Group F2F meetings

Working methods

The TTWG currently operates different working methods for development of TTML2 and IMSC than for WebVTT.

TTML2 and IMSC and other documents are generally discussed in weekly telephone conferences. Work on WebVTT tends to happen in github and on the reflector.

Those products that are managed with tracker are listed at products. Users of these products may identify issues within them, either before or after publication, and may wish to contribute developments, ideas or editorial changes to a product prior to publication.

Changes to products

All proposed changes must be recorded as issues which are managed on github following repos: imsc, imsc-tests, PQ HDR in PNG, ttml1, ttml2, ttml-webvtt-mapping and webvtt.

Issues and Pull Requests

For bugs it is sufficient simply to raise an issue on github following repos: imsc, ttml1, ttml2, ttml-webvtt-mapping and webvtt.

TTWG Dashboard issues/pull requests on github for following repos: imsc, ttml1, ttml2, ttml-webvtt-mapping and webvtt.

Issues that need further elaboration can be documented and discussed by adding a page link here.

  • Issue 151 Clarify syncbase in SMPTE continuous mode

New features

For development of new TTML features it is preferred that the features are described within #Change Proposals and discussed prior to approval either by teleconference meetings, usually held weekly, at face to face meetings, held on an occasional basis, or on the public email reflector which is archived here. The Chair approves change proposals according to the W3C consensus policy.


For examples of TTML styling (including IMSC and TTML2) that is currently difficult to map into CSS see TTML/CSSRequirements.


Where actions are assigned to members in order to progress an issue or a change proposal these are recorded in the tracker.

Mail archive

Emails to the group's public email address can be viewed in the public archive. Members can view emails to the group's member-only address in the member archive.

Change Proposals

As we look to creating the next version of the specification to address open issues we will adopt a lightweight change proposal mechanism. These change proposals are recorded in the Change Proposal Index

Evidence of Wide Review

TTML Action Items

Action items are tracked.

TTML Documentation

You can help by providing Timed Text Documentation.