Host header permitted under HTTP 1.0

Is the host header permitted for HTTP 1.0 requests or should the following request return a 400?

GET / HTTP/1.0
Host: myhost

In other more RFC-like words, MAY a host header be sent in a HTTP 1.0 context or MUST it NOT be sent (and should be answered with a respective 400 response)? The question is specifically not about CAN.