Comment 1 for bug 1849644

It isn't mandatory to use a standardized subprotocol, all that's required is that the client & server agree

  https://developer.mozilla.org/en-US/docs/Web/HTTP/Protocol_upgrade_mechanism

  "The subprotocols may be selected from the IANA WebSocket Subprotocol Name Registry or may be a custom name jointly understood by the client and the server."

QEMU used/required 'binary' because that is what noVNC used when the QEMU websockets code was first implemented.

It appears that noVNC was changed though to not send a "binary" subprotocol in

  commit f8318361b1b62c4d76b091132d4a8ccfdd2957e4
  Author: Pierre Ossman <email address hidden>
  Date: Sat Oct 14 12:45:56 2017 +0200

    Remove wsProtocols setting

    It isn't in use anymore since we deprecated support for Base64 mode.

From QEMU's POV looks like we'll need to tweak code to treat 'binary' and no sub-protocol as being equivalent.