One interesting use case for SXG is to allow decentralised and offline websites, since the site's data can be tied to a key/certificate/domain without having to be downloaded from a specific server. As an example, the IPFS project is already trialling the technology:
tying it to a domain name (which is the typical use of the URL) breaks the web though. i could understand if the key is used to show that the origin is a twitter account handle or something, but breaking the semantics of the domain by signing the content doesn't make any functional sense. Other than putting lipstick on a pig (AMP) of course
https://github.com/ipfs/in-web-browsers/issues/121