Use case (content author): Lazy-load the map viewer

[Malvoz]

This issue is for discussion of the use case “Lazy-load the map viewer”, its examples & list of required capabilities.

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This use case is about lazy-loading fetches initiated by the map element (or rather any of it's child elements, perhaps only those fetching tilesets) when the map viewer is out of viewport. Not to be confused with lazily loading tiles as described in Capability: Load additional map tiles when they pan into view.

Related web specification:
https://html.spec.whatwg.org/multipage/urls-and-fetching.html#lazy-loading-attribute