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Cloudmersive API is a free Machine Learning API that requires apiKey authentication, is served over HTTPS and supports CORS, so it can be called from the browser.
Details from cloudmersive.com.

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The Cloudmersive API is provided by Cloudmersive (cloudmersive.com).
Yes. The Cloudmersive API is listed in the Machine Learning 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 Cloudmersive API uses apiKey authentication, so you need to register for credentials before making requests.
Yes. The Cloudmersive API supports CORS, so front-end JavaScript can call it directly.
Yes. The Cloudmersive API is served over HTTPS, so it's safe to use in production.