
Spain Government Open Data.
Open Government, Spain API is a free Government API that requires no API key, is served over HTTPS and has unconfirmed CORS support.
Plataforma de datos abiertos del Gobierno de España
Details from datos.gob.es.

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The Open Government, Spain API is provided by datos.gob.es (datos.gob.es). Plataforma de datos abiertos del Gobierno de España
Yes. The Open Government, Spain API is listed in the Government 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 Open Government, Spain API requires no authentication, so you can start making requests without signing up.
CORS support for the Open Government, Spain API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Open Government, Spain API is served over HTTPS, so it's safe to use in production.