SSL Domain Health Check
SSL certificate validity, domain WHOIS status, and DNS record checks for any domain

Deep SSL/TLS server analysis with A+ to F grading.
SSL Labs API is a free Security API that requires no API key, is served over HTTPS and has unconfirmed CORS support.
A command-line reference-implementation client for SSL Labs APIs, designed for automated and/or bulk testing.
Details from github.com.

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Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
SSL certificate validity, domain WHOIS status, and DNS record checks for any domain
Threat intelligence: incident clusters, validated IOCs, CVEs, ransomware leak-site victims
The SSL Labs API is provided by ssllabs (github.com). A command-line reference-implementation client for SSL Labs APIs, designed for automated and/or bulk testing.
Yes. The SSL Labs API is listed in the Security 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 SSL Labs API requires no authentication, so you can start making requests without signing up.
CORS support for the SSL Labs API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The SSL Labs API is served over HTTPS, so it's safe to use in production.