[dangrossman]

The problem is that the browser is not the only client involved. Corporate/country/personal proxies, firewalls and antivirus software all sit in between some servers and clients, some of them still do not handle HTTP/1.1 correctly, and many of them will alter your headers in transit.

As your StackOverflow link mentions, Squid, a widely used proxy, is still only fully implemented for HTTP/1.0.

[pornel]

Corporate/country/personal proxies, firewalls and antivirus software don't perform redirects themselves, so they are irrelevant for status 303 support.

Today all clients that send HTTP/1.0 version support more than HTTP/1.0, e.g. HTTP/1.1 Host header is basically mandatory on the web.

HTTP/1.1 compliance requires some harder things like pipelining support, and full HTTP/1.1 caching proxy has a lot of hairy stuff to deal with (strong/weak cache validators, stitching of partial responses, Vary support, etc.).

You can have quite decent HTTP/1.1 implementation that still doesn't deserve to be called full HTTP/1.1.