
Check if any website is down, from two continents; live status, uptime history and TLS expiry.
Is This Site Down? API is a free Development API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.
A free, open, cookieless HTTP API that checks whether a website is up or down from an independent server and returns JSON. No key, CORS enabled, 20 requests per minute per IP. Attribution appreciated.
Details from downforjustmeoreveryone.com.

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Convert JSON to JSONP (on-the-fly) for easy cross-domain data requests using client-side JavaScript
The Is This Site Down? API is provided by Is This Site Down? (downforjustmeoreveryone.com). A free, open, cookieless HTTP API that checks whether a website is up or down from an independent server and returns JSON. No key, CORS enabled, 20 requests per minute per IP. Attribution appreciated.
Yes. The Is This Site Down? 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.
No. The Is This Site Down? API requires no authentication, so you can start making requests without signing up.
Yes. The Is This Site Down? API supports CORS, so front-end JavaScript can call it directly.
Yes. The Is This Site Down? API is served over HTTPS, so it's safe to use in production.