When I was working on mod_pagespeed I wrote some about how we decided to parse the UA: https://www.jefftk.com/p/why-parse-the-user-agent https://www.jefftk.com/

When I was working on mod_pagespeed I wrote some about how we decided to parse the UA: https://www.jefftk.com/p/why-parse-the-user-agent https://www.jefftk.com/p/why-parse-the-user-agent (Summary: feature detection requires more round trips, and slows down pages.)