> >some images on the W3C site.  None of these can effectively use 
> >q factors as the common browsers do not allow the users to configure
> >them, relying on positional fallback instead.
> 
> They do for Accept-Language and there are quite a number of sites that

Most servers understand q factors, but they can't be effectively used
as most browsers don't provide any mechanism to specify them.

> use Accept-Language for content negotiation.

They are vanishingly rare on sites run by companies whose first language
is English or Chinese.