HTTP/1.0 was described in RFC 1945.
HTTP/1.0 does not have some features that are today de-facto required on the Web, such as the
Host header for virtual hosts.
However, HTTP clients and servers sometimes still declare they use HTTP/1.0 if they have incomplete implementation of the HTTP/1.1 protocol (e.g. without chunked transfer encoding or pipelining), or compatibility is considered more important than performance (e.g. when connecting to local proxy servers).
GET / HTTP/1.0 User-Agent: example/1 HTTP/1.0 200 OK Content-Type: text/plain Hello