Just found a revelatory discussion. The summary of Bjoern Hoehrmann is especially interesting with regards to that topic.
While skimming through the new HTML5 WebSocket draft, I noticed the following exemplar HTTP message demonstrating the client message of a WebSocket handshake:
GET /demo HTTP/1.1
Sec-WebSocket-Key2: 12998 5 Y3 1 .P00
Sec-WebSocket-Key1: 4 @1 46546xW%0l 1 5
To me this looks non conforming to the HTTP spec due to the lack of an indicator that the request contains a message body.
Quoting the 4th paragraph of section 4.3 of RFC2616:
The presence of a message-body in a request is signaled by the inclusion of a Content-Length or Transfer-Encoding header field in the request's message-headers. A message-body MUST NOT be included in a request if the specification of the request method (section 5.1.1) does not allow sending an entity-body in requests. A server SHOULD read and forward a message-body on any request; if the request method does not include defined semantics for an entity-body, then the message-body SHOULD be ignored when handling the request.