These types should be supported by all backends, as much as each backend is technically capable of supporting:

• FormData (for request only)
• Blob
• Document (for response only)
• Text (String)
• JSON

TBD: ArrayBufferView/ArrayBuffer?

All expected response types should be able to be explicitly declared on the Request, so that they can be optimized by the underlying primitive (see #2609). Request body types won't be explicitly declared.