Great idea, and I’d be willing to participate in any discussion, say on a W3C mailing list, to take it further.

It’s worth noting the work of the Spatial Data on the Web Working Group[1]

There are also some similarities with the “Geo” microformat [2], which relies on the visible display of coordinates, and that mode might even be a good fallback when a map service is not available.

We’d need a way to specify the CRS (Coordinate Reference System), assuming WGS84 is the default – not least so we can say that a point is on the Moon, or Mars, or wherever, and have the browser display an appropriate map in such cases.

The zoom level might be adjusted according to the user’s location: “how to get here” varies if you’re three blocks or three miles away, for example. It should also be possible to instruct the browser to put a marker on the map for the user’s current location; and/ or to enter “directions” mode and show a route.

It might also be sensible to allow the author to specify the coordinates of a bounding box (say, top-right and bottom left), and have the browser display a map zoomed according to the size of the available viewport.

[1] https://www.w3.org/2015/spatial/wiki/Main_Page

[2] https://en.wikipedia.org/wiki/Geo_(microformat)