Library info on CDNJS.
CDNJS API is a free Development API that requires no API key, is served over HTTPS and has unconfirmed CORS support.

Unified API suite · now live
Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
Free hosting for HTML, JSON, key-value, files, multi-file sites with short URLs and TTL retention
Chart image API: turn a URL into a PNG or SVG chart (bar, line, area, scatter, pie, donut, stacked)
Yes. The CDNJS 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 CDNJS API requires no authentication, so you can start making requests without signing up.
CORS support for the CDNJS API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The CDNJS API is served over HTTPS, so it's safe to use in production.