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Earlier, encryption method TLS 1.2 or greater was planned to be mandatory as part of the protocol. However, in lieu of consensus for mandatory TLS, an optional unencrypted mode exists in addition to required support of an encrypted mode. Some implementations, such as Firefox, have stated that they will only support HTTP/2 when it is used over an encrypted connection.
The Question: Why wouldn't it be great, if HTTP/2 would only allow communication via TLS?