Why do browser user-agent strings always include Mozilla/5.0?

Why do browser user-agent strings always include Mozilla/5.0? What does “Mozilla/5.0” in user agent string signify? History of the browser user-agent string Quick answer Because website developers may output special features when they detect a browser (Mozilla in this case). When other browsers also support those good features, they try to mimic Mozilla so the site outputs the same content instead of a degraded version.