User Agent Parser

A user agent string is an HTTP header sent by browsers that identifies the browser, operating system, and rendering engine. While historically used for browser detection, modern best practice is feature detection over UA sniffing.

Historical compatibility. Early browsers claimed to be "Mozilla" to bypass Netscape-only checks. Later, WebKit-based browsers added "Safari" to pass Apple-only checks. This frozen UA string convention persists to this day - it's a fascinating piece of web history.

Browser wars history. To avoid being blocked by servers checking for 'Mozilla', every browser claims to be Mozilla. Chrome includes 'Safari' and 'KHTML, like Gecko' tokens for compatibility. This is why modern UA strings look like they're identifying 5 different browsers.

No. Feature detection (checking if a specific API exists) is more reliable than UA parsing. UA strings can be spoofed, are increasingly frozen by browsers for privacy, and don't reflect actual capabilities. Use feature detection with progressive enhancement for cross-browser compatibility.

Absolutely. User agent strings are easily spoofed by bots, scrapers, or privacy-focused browsers. Never rely solely on UA parsing for security decisions. For definitive browser detection, use client-side feature detection (e.g. Modernizr).