HTML horizontal review: Modules

[hober]

Subtask of #412.

PRs to review:

• JSON module support whatwg/html#4407 (JSON Modules)

[hober]

Re: whatwg/html#4407:

The TAG has already reviewed JSON modules, in #375.

Also, we note that this feature is absent from the HTML Living Standard (as this PR was rolled out in whatwg/html#4943), but it is present in the July 2019 Review Draft. When the HTML WG moves forward with the July 2019 RD, this feature should be marked at risk in the Review Draft, ideally with rationale & links to further info.