What is q=0.5 in Accept* HTTP headers?

Question

Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7

Answer 1

Accept-languages quality parameter can be understood like this:

by attaching Accept-Language: pl_PL;q=0.8, en_US;q=0.2, header to the request we communicate to the server following message

Server, please provide me Polish translation of this website, but please do that only if its quality is >= %80% of related-quality-factor. In case quality is <80% - I do not mind getting the English version, because it is irrelevant for me since I speak English anyway.

So for example, as a web page content provider we might introduce Accept-Language header parsing in such a way, that our website is suitable for foreigners speaking no English at all (then even translated navigation would be helpful (pages having translated only the navigation, lets say, have q=0.1), where fully translated content has q=0.9 and fully translated content and verified by native speakers speaking both original language and translated language content might have q=0.99 - because this is the only possible situation when meaning of the content is fully transduced)