I think what the GP is describing isn't about client-server negotiation, though, but rather something that happens purely client-side for already-delivered reso

I think what the GP is describing isn't about client-server negotiation, though, but rather something that happens purely client-side for already-delivered resources, within e.g. the system for loading the right subtitle track embedded into a playing video. AFAIK, there's no standard for allowing client-side media-playback frameworks to probe language preferences with quality values out of the browser they're running in. The navigator.languages property (https://developer.mozilla.org/en-US/docs/Web/API/Navigator/languages https://developer.mozilla.org/en-US/docs/Web/API/Navigator/l...) doesn't expose qvalues for some reason. And even if there was, there's no way, in any media container format I know of, of annotating subtitle tracks with quality values — so there's no way of saying that a given track really sucks and should be deprioritized relative to the user's preference ordering, but still used if there's no other choice.