HTTP pipelining is a technique in which multiple HTTP requests are written out to a single socket without waiting for the corresponding responses. Pipelining is only supported in HTTP/1.1, not in 1.0.
Only idempotent requests should be pipelined, such as GET and HEAD requests. POST requests should not be pipelined. Requests on a new connection should not be pipelined either, since it has not yet been determined if the origin server (or proxy server) supports HTTP/1.1. Hence, pipelining can only be done when reusing an existing (keepalive) persistent connection.
HTTP pipelining requires both the client and the server to support it. HTTP/1.1 conforming servers are required to support pipelining. This does not mean that servers are required to pipeline responses, but that they are required not to fail if a client chooses to pipeline requests.
 Implementation status
 Implementation in web servers
Implementing pipelining in web servers is a relatively simple matter of making sure that network buffers are not discarded between requests. For that reason, most modern web servers handle pipelining without any problem.
Exceptions include IIS 4 and reportedly 5.
 Implementation in web browsers
Internet Explorer as of version 7 doesn't support pipelining.
Mozilla Firefox 3.0 supports pipelining, but it's disabled by default. It uses some heuristics, especially to turn pipelining off for IIS servers. Instructions for enabling pipelining can be found at Firefox Help: Tips & Tricks. Camino does the same thing as Firefox.
 Implementation in web proxies
Most HTTP proxies do not pipeline outgoing requests.
Some versions of the Squid web proxy will pipeline up to two outgoing requests. This functionality has been disabled in the latest versions. Squid supports multiple requests from clients.
The Polipo proxy pipelines outgoing requests.