Signed HTTP Exchanges require SSL certificates with the CanSignHttpExchanges extension. DigiCert is currently the only provider of these certificates. It would be wonderful if support for this extension could be incorporated into Let’s Encrypt certificate generation.
Signed HTTP Exchanges (jump to certificate requirements)
Signed HTTP Exchanges Implementation Checkpoints
Reference implementation of Signed HTTP Exchanges format generator
Google write-up about HTTP Signed Exchanges
DigiCert with CanSignHttpExchanges extension