Are you trying to check for the existence of a specific feature? Browser makers do pay attention to bug reports, and the analysis may hint about other workarounds for the bug. If the problem seems uncommon, it's worth checking if this bug has been reported to the browser vendor via their bug tracking system ( Mozilla WebKit Blink Opera). Also, experts, or people with another point of view, can give you ideas for working around the bug. Look, or ask, in specialized forums: you're unlikely to be the first to hit this problem. Are you trying to work around a specific bug in some version of a browser? Start by trying to identify why you want to do it. When considering using the user agent string to detect which browser is being used, your first step is to try to avoid it if possible. Experimental Feature-Policy: xr-spatial-tracking.Experimental Non-standard Feature-Policy: unsized-media.Experimental Non-standard Feature-Policy: unoptimized-images.Experimental Non-standard Feature-Policy: sync-xhr.Experimental Feature-Policy: speaker-selection.Experimental Feature-Policy: screen-wake-lock.Experimental Feature-Policy: publickey-credentials-get.Experimental Feature-Policy: picture-in-picture.Experimental Non-standard Feature-Policy: oversized-images.Experimental Feature-Policy: magnetometer.Experimental Non-standard Feature-Policy: legacy-image-formats.Experimental Non-standard Feature-Policy: layout-animations.Experimental Feature-Policy: encrypted-media.Experimental Feature-Policy: document-domain.Experimental Feature-Policy: ambient-light-sensor.Experimental Feature-Policy: accelerometer.Reason: CORS preflight channel did not succeed.Reason: CORS header 'Origin' cannot be added.Reason: Credential is not supported if the CORS header 'Access-Control-Allow-Origin' is '*'.Reason: Multiple CORS header 'Access-Control-Allow-Origin' not allowed.Reason: CORS header 'Access-Control-Allow-Origin' missing.Reason: missing token 'xyz' in CORS header 'Access-Control-Allow-Headers' from CORS preflight channel.Reason: expected 'true' in CORS header 'Access-Control-Allow-Credentials'.Reason: Did not find method in CORS header 'Access-Control-Allow-Methods'.Reason: invalid token 'xyz' in CORS header 'Access-Control-Allow-Methods'.Reason: invalid token 'xyz' in CORS header 'Access-Control-Allow-Headers'.Reason: CORS request external redirect not allowed.Reason: CORS header 'Access-Control-Allow-Origin' does not match 'xyz'.Experimental CSP: require-trusted-types-for.Non-standard Deprecated CSP: require-sri-for.Non-standard Deprecated CSP: plugin-types.Deprecated CSP: block-all-mixed-content.Experimental Sec-CH-UA-Platform-Version.Experimental Sec-CH-UA-Full-Version-List.Non-standard Deprecated Large-Allocation.Non-standard Deprecated Accept-CH-Lifetime.