Blockchain-based blogging and social media website.
Steem API is a free Blockchain API that requires no API key, is only available over HTTP and does not support CORS, so it should be called from a server.
Steem Developer Documentation.
Details from developers.steem.io.

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The Steem API is provided by Steem Developer (developers.steem.io). Steem Developer Documentation.
Yes. The Steem API is listed in the Blockchain 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.
No. The Steem API requires no authentication, so you can start making requests without signing up.
Not directly. The Steem API does not allow CORS, so call it from a server, serverless function or proxy instead of front-end JavaScript.
No. The Steem API is only available over plain HTTP, which browsers block on HTTPS sites. Call it from a server instead.