The as attribute is defined as taking a https://fetch.spec.whatwg.org/#concept-potential-destination.*

https://html.spec.whatwg.org/#match-preload-type however returns false for a number of destinations.

We should probably make it clearer to web developers what they can preload.

cc @noamr

*Ouch. It seems that definition has the member/value type conflict going on.