There are 2 repositories under mapml topic.
Map Markup Language is hypertext for Web maps, like HTML is hypertext for Web pages https://maps4html.org/MapML/spec/
A custom <mapml-viewer> and <layer-> element suite
Proposed spec for dynamic slippy maps in HTML: https://maps4html.github.io/HTML-Map-Element/spec/
Maps for HTML MapML Extension / plug-in development fork of GeoServer
MapML documentation on MapServer
A clone of validator.nu for Map Markup Language