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 

But curious to know what is q=0.9,*/*;q=0.8

Answer 1

Each media-range MAY be followed by one or more accept-params, beginning with the "q" parameter for indicating a relative quality factor. The first "q" parameter (if any) separates the media-range parameter(s) from the accept-params. Quality factors allow the user or user agent to indicate the relative degree of preference for that media-range, using the qvalue scale from 0 to 1. The default value is q=1

The information is available here

A nice explanation can be found here as well.

Answer 2

The Accept header list is first split at , then at ; for additional parameters per entry. So, the list in your example splits down to text/html, application/xhtml+xml, application/xml;q=0.9 and */*;q=0.8. The q= parameter on each entry indicates to the server the degree of preference for that media-type. It defaults to its maximum value of 1, if it is missing (like in the first 2 entries). The last entry of */*;q=0.8 indicates to the server, that eventually any content-type would be acceptable but would be less preferable than the others listed. Otherwise, the server might decide to send no content at all, because the client would not "accept" it anyway.