Define schema / model for MapML content of <layer> element

[prushforth]

Define expected content of <layer> element, and the way it should be marked up. Note that iframe uses a srcdoc attribute, and keeps the iframe element empty.

[Malvoz]

Define expected content of <layer> element

That resource requires authorization, is this the same publication? https://docs.opengeospatial.org/per/18-023r1.html

[prushforth]

Yes, fixed I think. Thanks!