Hidden features of HTTP

Question

What hidden features of HTTP do you think are worth mentioning?

By hidden features I mean features that already are part of the standard but widely rath

Answer 1

204 No Content

I thought 204 was just if you have no content to display, but the spec looks like there is additional behavior that the user agent "not change its document view."

According to HOWTO: Configure Apache to Return a HTTP 204 (No Content) for AJAX

FWIW, Google actually does something similar. Each time a user clicks on a link in their search results, Google pings itself to record the click; the response code from the ping is an HTTP 204.

Also, 204 No Content proposes this is a good technique for "web bugs" or "beacons" if you want to save on every last byte of network traffic you can.