Integration with Fetch / Fetch Metadata Request Headers

[Malvoz]

https://fetch.spec.whatwg.org/#concept-request-destination:

Features need to have the relevant values defined in their respective standards.

Fetch Metadata Request Headers expose metadata about requests, for example, a requests's destination in the Sec-Fetch-Dest request header field's value. This value should be defined for all elements that fetch resources, e.g. <layer src>, <tile src>, etc.

The Portals spec can serve as an example of such integration: https://wicg.github.io/portals/#fetch-metadata

https://github.com/Maps4HTML/MapML/issues/22 is related to the Sec-Fetch-Mode header.