Secure User-Agent String Lookup JSON API.
Userstack API is a free Development API that requires OAuth authentication, is served over HTTPS and has unconfirmed CORS support.
Take control of your user experience with userstack, a lightweight User-Agent lookup JSON and XML API for real-time device, browser, and OS detection.
Details from userstack.com.

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The Userstack API is provided by userstack (userstack.com). Take control of your user experience with userstack, a lightweight User-Agent lookup JSON and XML API for real-time device, browser, and OS detection.
Yes. The Userstack API is listed in the Development category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The Userstack API uses OAuth authentication, so you need to register for credentials before making requests.
CORS support for the Userstack API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Userstack API is served over HTTPS, so it's safe to use in production.