Revise and simplify top level explainer.

[prushforth]

No description provided.

[prushforth]

I've been trying to update this explainer in light of @zcorpan's 2020 review. That review, and the explainer itself, didn't have access to research that was done by @Malvoz and @nchan0154 for the 2020 workshop, and later research by @zcorpan and others. I believe that those two project provide the most authoritative (to date) rationale for standardizing web maps, and I would like to include them as part of the "problem statement" (Step 0. Describing the problem: What specific problem are you trying to solve. Who are you trying to solve it for?). The explainer should keep it simple and short

Reviewing those resources, I found some guidance on proposing new platform features, and what constitutes a good problem statement and list of consequent requirements, and it seems quite simple, actually. I started copying the form of requirements enumeration there and it started to get interesting. I'm not sure if it fits into the "explainer" format, but if you have any comment on this list, I would love to hear it. I will probably adapt it a bit to the explainer introduction.

(1) A search crawler wants to index web pages according to the location(s) of information in the page.
Requirements:
- Pages must be able to express location(s) so they are rendered semantically in a projected 2D CRS visual context.
- The coordinates used to express location(s) must be related to the surface of the earth in a defined manner.
- The search crawler should not have to “execute” the page scripts in order to “understand” the spatial content / context.

(2) A screen reader user wants to know what locations represented on a web map are closest to them, or are considered most “important” to the web map author.
Requirements:
- Must provide a viewport in which real-world coordinates drive content rendering
- Must be able to locate the user, and to automatically move / pan / zoom the map viewport to the user’s location
- Must be able to sort the represented map locations by distance from the user’s location, or from the map viewport center, if the user’s location is not used
- Must be able to present sorted features to the accessibility technology accordingly

(3) A web developer wants to represent, style, and programmatically manipulate real-world locations in their page using HTML, CSS and JavaScript, according to WCAG
Requirements:
- Must be able to provide a viewport in which real-world coordinates drive content rendering
- Must have declarative content that encodes its location

(4) A government data provider wants to share authoritative, accessible and interoperable map location information on the web as a service, so that all users have access to the most up to date information (say, in an emergency)
Requirements:
- A server must be able to share web map content that encodes location information in a standard and semantic format that can be used via a simple URL
- Developers must be able to readily integrate the shared location information into a web map via its URL

[zcorpan]

+1 on the above format.