1.3. Languages and Typesetting
https://drafts.csswg.org/css-text-3/#content-language

Note: Authors can tag content using the global lang attribute in HTML, the universal xml:lang attribute in XML, and the HTTP Content-Language header for content served over HTTP.

The i18n WG recommends always using document-internal declarations, even if the appropriate language is contained in the HTTP Content-Language header. Obtaining the language from the HTTP header should only be a last resort fallback, and even then it's problematic. See https://www.w3.org/International/questions/qa-http-and-lang (esp. https://www.w3.org/International/questions/qa-http-and-lang.en#http).

I think, therefore, the note should be reworded so that it doesn't imply that HTTP declarations are of equal validity to the lang attributes. Really it should say something like: authors can use the lang or xml:lang attributes, and some browsers in some cases may be able to extract something useful from the HTTP information if those are lacking. Personally, i'd be inclined to just drop the mention of the HTTP Content-Language completely.

(I was actioned to send this comment by the i18n WG)