HTTP-Headers Status Diagram
thoughtpad.net
thoughtpad.net
This concerns only web server implementors... and even then, a lot of web servers simply don't need to support the full set of operations.
I admit I went a bit overboard with saying "can completely ignore the lion's share" but I obviously didn't mean you should go out of your way to skip the absolute basics that caching, content negotiation, error codes etc. are.
I wasn't "advising" against learning HTTP, I just think lots of web developers get away with not learning much about it (I'm NOT saying that's a good thing.)
edit: Damn, the more I reread my previous post the stupider it sounds :|
But certainly useful.