[w3ctag/design-reviews] HR review of IMSC 1.2 FPWD (#474)

Hello TAG!

I'm requesting a TAG review of IMSC 1.2 FPWD, specifically the [delta relative to IMSC 1.1](https://www.w3.org/TR/ttml-imsc1.2/#substantive-changes-summary), of which the main substantive change is the addition of support for the [`#font`](https://www.w3.org/TR/ttml-imsc1.2/#text-font-source) feature.

This feature allows the content author to specify an external font resource to be used when presenting the document, with attributes similar to the descriptors that can be specified with the CSS `@font-face` rule.

  - Explainer¹ (minimally containing user needs and example code): Closest we have is https://github.com/w3c/tt-reqs/issues/15

  - Requirements: https://w3c.github.io/imsc-vnext-reqs/ in particular the requirement for [font support](https://w3c.github.io/imsc-vnext-reqs/#font-support)
  - Specification URL: [IMSC 1.2 FPWD](https://www.w3.org/TR/2019/WD-ttml-imsc1.2-20191128/)
  - Tests: https://github.com/w3c/imsc-tests/tree/master/imsc1_2

  - Security and Privacy self-review²: This is largely unchanged since IMSC 1.1 however note the privacy review issue raised against TTML2, from which some new features are referenced, at w3c/ttml2#1189
  - GitHub repo (if you prefer feedback filed there): https://github.com/w3c/imsc/issues/

  - Primary contacts (and their relationship to the specification): 
      - [Pierre-Anthony Lemieux] (@palemieux ), IMSC Editor, representing MovieLabs
      - [Nigel Megitt] (@nigelmegitt), TTWG Chair, BBC
  - Organization(s)/project(s) driving the specification: [organization and/or project name] 
  - Key pieces of existing multi-stakeholder review or discussion of this specification: 
  - External status/issue trackers for this specification (publicly visible, e.g. Chrome Status): None

Further details:

  - [x] I have reviewed the TAG's [API Design Principles](https://w3ctag.github.io/design-principles/) _but it is not relevant since this is not an API specification_
  - Relevant time constraints or deadlines: we would appreciate an expedited review of this small delta please.
  - The group where the work on this specification is being done: W3C TTWG
  - Major unresolved issues with or opposition to this specification: None
  - This work is being funded by: TTWG group participants

You should also know that...

The FPWD was published on 2019-11-28 and the feature delta is small. 

The main substantive requirement was to allow inline images such as company logos to be inserted into text. By allowing external font resources to be specified, the subtitle or captions document author can have some reassurance that the desired image is present in the selected font. Two image mechanisms are feasible: 1. A fallback image in the font file, e.g. the text "W3C" might be replaced inline with an SVG image of the W3C logo; 2. The Unicode private use area of code points can be used to define bespoke images.

We'd prefer the TAG provide feedback as (please delete all but the desired option):

  🐛 open issues in our GitHub repo for **each point of feedback**



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