Add requirements to VideoFrame metadata registry definition

[chrisn]

Following the discussion in #559, this PR covers further changes to adding, changing, and deprecating registry entries.

[aboba]

Lines 40-42: You might say, "The specification defining the metadata entry must provide clearly defined semantics." That is, the semantics should be defined in the specification, not the registration request.

[chrisn]

Thanks, I have pushed an update to clarify this.

[tidoust]

Three questions:

1. Does this need to be restricted to W3C specifications?
2. Should this be restricted to normative specifications?
3. What does it mean for a W3C specification to have "reached consensus in the originitaing Working Group"?

For 1., I realize that it's hard to come up with an explicit set of requirements for a specification to be considered good enough. A document that comes to mind are the considerations the Director takes into account when evaluating normative references from W3C documents. These considerations include considerations such as:

• Is the specification produced by a group that follows the OpenStand principles?
• Is the specification available in English and at no cost?
• Are the technologies in the referenced parts available under terms that are compatible with the W3C Royalty-Free licensing requirements?

For 2., I'm wondering whether the group is ok with specifications on the Note track, as done for codec registrations but perhaps not such a good example to follow?

For 3., a status on the Recommendation track (e.g. Candidate Recommendation), would make things clearer. Also, I'm thinking that the group might want to add entries even when a spec is still under development?

I'm having a hard time coming up with a proposal that is not convoluted, so perhaps it's fine to leave this a bit under specified. If the group wanted to open the registry to non-W3C specs, the following wording could be considered:

Suggested change

	3. Each metadata entry must be defined by a W3C specification that has
	reached consensus in the originating Working Group.
	3. Each metadata entry must be defined in an established standard with licensing terms that are consistent with royalty-free implementation of W3C Recommendations. The Media Working Group will evaluate candidate specifications following [considerations used when evaluating normative references from W3C specifications](https://www.w3.org/2013/09/normative-references) [[NORMATIVE-REFERENCES]].

[chrisn]

Thanks @tidoust, my thoughts on your questions:

1. We discussed limiting to W3C specifications in [WebCodecs VideoFrame metadata registry] Introduce VideoFrame metadata #559 (comment), my interpretation is partly as a way to ensure compatible licensing
2. My understanding is that metadata entries should be normative, but they may be optional to implement, per [WebCodecs VideoFrame metadata registry] Introduce VideoFrame metadata #559 (comment)
3. I expect that we will want to add entries in draft state. I saw consensus here as: Media WG having consensus to add the registration, and the originating WG having a draft spec to add and consensus on its purpose and scope, it's OK if details are still in development ([WebCodecs VideoFrame metadata registry] Introduce VideoFrame metadata #559 (review))

[youennf]

LGTM.
With regards to the process, I think keeping it a bit unspecified until we exercise it seems ok.
In general, I would think that metadata entries being normative, they should follow the REC track.
The restriction to W3C specifications (but they might refer to non W3c specs themselves) is fine for now as well, given spec authors will anyway need to interact deeply with the Media WG.

[dalecurtis]

Are we ready to merge this one?

[chrisn]

I think so, @padenot is this OK from your perspective?

[chrisn]

@padenot I'd like to merge this, as we now have incoming proposals to add to the registry. Thanks!

[padenot]

Sure, sorry for missing this, go ahead!