There is support for extensions, but they're, well, extensions. The only thing the protocol specifies is that a compliant implementation must pass-through unchanged any block it doesn't understand.
Compare with HTTP/1.1 where for instance the entire content negotiation mechanism is optional and clients need to be able to deal with it not being available.
> There is support for extensions, but they're, well, extensions.
So down the line, it will be pretty much exactly like HTTP 1.0 and 1.1 then.
Good to hear someone thought this thoroughly through before creating a mega-complex protocol unimplementable by most industry-grade engineers, which will also need to be debugged and maintained for all internet-eternity.
Compare with HTTP/1.1 where for instance the entire content negotiation mechanism is optional and clients need to be able to deal with it not being available.