What is a User Agent (UA) string?

A User Agent string is a line of text sent by your web browser in HTTP request headers (User-Agent:) to identify the browser family, version number, host operating system, and rendering engine to web servers.

What are User-Agent Client Hints (UA-CH)?

User-Agent Client Hints are modern HTTP headers and JavaScript APIs (navigator.userAgentData) that provide browser and device information in a privacy-preserving, structured manner, replacing legacy frozen User-Agent strings.

Why do modern Chrome User-Agent strings say "Mozilla/5.0"?

Historical compatibility. In the early days of the web, webmasters wrote scripts targeting Mozilla (Netscape). When newer browsers like Chrome and Safari emerged, they included "Mozilla/5.0" so servers would send them full-featured web pages.

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