HTTP 1.0 vs HTTP 1.1

HTTP 1.1 has key differences and improvements, but version 1.0 is still in use by old clients.

Features HTTP 1.0 HTTP 1.1
Connection Creates a new TCP connection for each request that should be closed after the server responds. This leads to big performance problems due to the cost of setting up a new TCP connection and closing it. Uses persistent connections and pipelined requests over the established connection. This has a positive impact on performance.
Host field Does not require a host header by specification. The Host header is required by the specification. It allows to route HTTP messages through proxy servers, and distinguish between different websites on the same server.
Authentication Only Basic Authentication. Digest and proxy authentications.
Caching Support caching only via the If-Modified-Since header. New If-Unmodified-Since, If-Match, If-None-Match conditional request headers and new Vary and Cache-Control response headers. A new 'entity tag'. If two resources are the same, then they will have the same entity tag.
OPTIONS New OPTIONS method. The client can use the OPTIONS method to determine the capabilities of the server. It's mostly used for Cross Origin Resource Sharing (CORS) requests.
100 Continue New 100 Continue status code to prevent clients from sending the request body if the server can process it due to size limits or authorization.