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API to estimate the carbon footprint of loading web pages.
Website Carbon API is a free Environment API that requires no API key, is served over HTTPS and has unconfirmed CORS support.
Details from api.websitecarbon.com.

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The Website Carbon API is provided by Website Carbon API (api.websitecarbon.com).
Yes. The Website Carbon API is listed in the Environment 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 Website Carbon API requires no authentication, so you can start making requests without signing up.
CORS support for the Website Carbon API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Website Carbon API is served over HTTPS, so it's safe to use in production.