[w3ctag/design-reviews] WG Revision: IMSC 1.3 (minor revision to IMSC 1.2) (Issue #1122)

palemieux created an issue (w3ctag/design-reviews#1122)

### Specification

https://www.w3.org/TR/2025/WD-ttml-imsc1.3-20250722/

### Explainer

_No response_

### Links

The WG's request for this TAG review: https://www.w3.org/2025/07/17-tt-minutes.html

TAG review of the previous version of this specification, if any: https://github.com/w3ctag/design-reviews/issues/474

A description of what has changed since our previous review: https://www.w3.org/TR/2025/WD-ttml-imsc1.3-20250722/#changes

- Japanese character set

A recommended supplementary character set for the Japanese language has been added based on feedback from ARIB, which is an organization of Japanese users.

https://github.com/w3c/imsc/issues/544

- Superscript/subscript text

Support for superscript/subscript text was added based on feedback from French users. This is merely permitting a feature that has been part of TTML2, of which IMSC is a profile, since its inception.

https://github.com/w3c/imsc/issues/583

- Image profile

The Image Profile specified in earlier versions of this specification is removed, following a survey of IMSC users.

https://github.com/w3c/imsc/issues/600

- Hypothetical Render Model

Following publication of the (IMSC Hypothetical Render Model)[https://www.w3.org/TR/imsc-hrm/] as a separate REC, the definition of the Hypothetical Render Model is removed from this document and replaced with a reference to the separate REC.

https://github.com/w3c/imsc/issues/582




### The specification

- [x] Follows the [Web Platform Design Principles](https://www.w3.org/TR/design-principles/).
- [x] Includes Security and Privacy Considerations sections based on answers to the [Security/Privacy Questionnaire](https://www.w3.org/TR/security-privacy-questionnaire/).

### Where and by whom is the work is being done?

- GitHub repo: https://github.com/w3c/imsc
- Primary contacts:
  - @palemieux (editor)
  - @nigelmegitt (WG co-chair)
  - @gkatsev (WG co-chair)
  - @himorin (Team contact)
- Organization/project driving the specification: TTWG


### Feedback so far

_No response_

### You should also know that...

Review requested no later than Sunday, October 19 2025, given the narrow range of changes.

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