It's not just about spoofing user agents. Take third party browsers for iOS for instance - they might be called Chrome or Opera but are really just UIWebViews. If they would add "Chrome" to their user agent string, this solution might break, serving incorrect polyfills. Today these browsers identify as "CriOS" and "OPiOS" to work around broken user agent detection. This can also be a problem with all web browsing happening in embedded webviews (feed readers, reddit apps, ...)