Identical <mapml-viewer> markup rendered by the
Leaflet build (left) and the MapLibre GL build (right). Each map runs
in its own iframe because both engines register the same custom-element
names. When an engine cannot honour the preferred resource, it falls
back to an alternative the author supplied.
Both maps declare projection="CBMTILE" and load the
same layer, whose Canada Base Map is offered in two
projections via a pair of <map-extent> elements: one
units="CBMTILE" (EPSG:3978 Lambert) and one
units="OSMTILE" (EPSG:3857 Web Mercator). Leaflet renders
the native CBMTILE extent (disabling the OSMTILE one); MapLibre GL, if
it cannot honour the Lambert projection, gracefully degrades to the
OSMTILE extent.