What is the correct way to do content negotiation?

Hi, if this has been asked before or if this is the wrong place to ask,
sorry.



Basically, what I'm wandering is, how am I supposed to do content
negotiation? At first I thought that I was supposed to use the HTTP_ACCEPT
header, but when I use your validator script at validator.w3.org to try to
test my website in different content types (by using the doctype option), it
doesn't change anything in the HTTP_ACCEPT header. Where is the information
located? Please help.. the requests look exactly the same to me.. =(




P.S. on a completely different topic; how do I change the options in your
validator script without it re-downloading my web page? Like for example if
I want to enable verbose output, how do I have it do this without it
downloading the page from my server again?