User Agent Viewer

Relying solely on UA strings for critical feature gates instead of capability checks, when UA strings can be spoofed or inaccurate, causing feature detection failures and broken functionality, requiring feature detection APIs (Modernizr, caniuse) rather than UA parsing for critical feature availability decisions.

Confusing Safari with Chrome due to WebKit tokens; check Version/Edg markers, when both browsers include WebKit tokens in UA strings, causing incorrect browser identification requiring inspection of version tokens (Version/ for Safari, Edg/ for Edge) to differentiate browsers accurately.

Ignoring device pixel ratio, causing blurry assets on HiDPI screens, when DPR affects image resolution requirements but isn't checked, causing pixelated or blurry images on high-DPI displays requiring DPR-aware asset selection and responsive image sizing for optimal visual quality.

Copying truncated UA strings; always copy the full value, when partial UA strings lose important identification information, causing incomplete browser/device identification requiring full UA string capture for accurate environment analysis and bug reporting.

Assuming one UA maps to one device; strings can be spoofed or altered, when UA strings can be modified by extensions, proxies, or user settings, causing incorrect device/browser assumptions requiring awareness that UA strings are not authoritative device identifiers.

Using UA strings for security decisions without additional verification, when UA-based security checks can be bypassed through spoofing, causing security vulnerabilities requiring multi-factor authentication and server-side validation rather than client-side UA checks for security.

Not refreshing environment data after viewport changes or device rotation, when viewport size, DPR, and screen resolution change with zoom or orientation, causing outdated environment information requiring page refresh or environment re-detection after layout changes.

Assuming UA parsing rules work identically across all parsing libraries, when different UA parsers use different detection logic, causing inconsistent results requiring understanding of specific parser behavior or using consistent parsing library across applications.

Using UA strings to determine mobile vs desktop without checking viewport or touch capabilities, when UA-based device type detection is unreliable due to spoofing or hybrid devices, causing incorrect responsive design decisions requiring viewport size and touch capability detection for accurate device type identification.

Ignoring privacy implications of UA string collection and sharing, when UA strings contribute to browser fingerprinting and user tracking, causing privacy concerns requiring consideration of privacy impact when collecting or storing UA strings in applications.

Not accounting for User-Agent Client Hints migration from UA strings, when modern browsers move toward Client Hints APIs instead of UA strings, causing future compatibility issues requiring understanding of Client Hints transition and planning for UA string deprecation.

Copying UA strings from documentation or examples without testing actual user agents, when example UA strings don't match real-world user agents, causing incorrect assumptions requiring testing with actual browser UA strings for accurate environment understanding.