The [docs have some description of the API](https://maps4html.org/web-map-doc/docs/api/mapml-viewer-api#definecustomprojectionoptions) (could use a refresh). We need to specify the `defineCustomProjection` method on the [`HTMLMapmlViewerElement` interface](https://maps4html.org/MapML/spec/#webidl-1786032469)
The docs have some description of the API (could use a refresh).
We need to specify the
defineCustomProjectionmethod on theHTMLMapmlViewerElementinterface