A User Agent refers to a browser or other application that can view or access web pages. (these apps are 'agents' that visit the site on a user's behalf). In the early days of the web, the only reliable way to detect which feature were supported by a browser was "user agent sniffing", or reading out the user agent string. Nowadays, we can use feature detection in both JavaScript and CSS instead. Because of this user agent sniffing, these strings contain many bits of text that mimic other, older browsers, to make sure the sites using this user agent sniffing continue to work properly.

The responsive design glossary explains the many terms used in responsive design and development. It's a practical reference for anyone getting started with responsive design and development.

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