As you know, HTTP/1.1 spec said that conforming clients SHOULD NOT open more than 2 concurrent connections to one host. This was defined back in 1997 and at that time it seemed reasonable to have 2 simultaneous connections for a client, and noting that HTTP/1.1 introduced persistent connections concept, people thought that 2 simultaneously opened reusable TCP/IP connections would be enough for general use.
However, everything changes. Broadband internet came to mass market and people started thinking that better parallel download could benefit the whole website or a webapp performance. The history started with IE5.01, which was opening two connections by default, but there was a way to configure the number. So if you had a really good internet connection, you could make websites load significantly faster.
By the time IE8 development started, broadband connections became a standard for home internet, so IE8 started opening 6 connections (if the bandwidth allowed – on the dialup or behind a proxy it will still open 2). So IE8 engineers did a smart move and introduced the world with a browser that seemed to load sites faster.
Needless to say, Firefox 3 decided to change the value as well, so now Firefox 3 has 6 as a default value for network.http.max-persistent-connections-per-server configuration setting. Good for Mozilla for copying stuff from IE again!
And now HTTPBis team (Julian Reschke) commits the change which states that in the forthcoming HTTP standard the maximum amount of concurrent requests is not limited even with “SHOULD NOT” clause :)
Thanks HTTPBis team!