Browsers of all types connect to the web, and the diversity is only increasing. It used to be acceptable to specify which browser parameters (e.g. browser maker or screen size) the user should adopt to properly view your content, but this is no longer the case. Instead, it is better to adapt the web page to the users' individual browsers, based on the presence or absence of specific features. This is where the concept of feature detection comes from.
Think of feature detection as a way of fingerprinting a web browser. Just like two people with the same name, two devices running the same browser will produce different fingerprints.