What is the purpose of the “q” values in the HTTP “Accept” request header?

Question

I have made a http request using Firefox.Now the request header shows the following:

Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
         


        

Answer 1

To add to @Robin479's correct answer, the accept header in the question is equivalent to,

Accept:
text/html;q=1,
application/xhtml+xml;q=1,
application/xml;q=0.9,
*/*;q=0.8

NOTE: The new line characters are just for better readability.