core-js is the more common of the two in our index: it runs on 16,657 websites against 66 for punycode, and 29 sites use both. Counts come from real scans, not vendor claims.
core-js is a modular standard library for JavaScript, with polyfills for cutting-edge ECMAScript features.
A robust Punycode converter that fully complies to RFC 3492 and RFC 5891, and works on nearly all JavaScript platforms.
29 sites run both core-js and punycode. That is 44% of the sites using punycode, the smaller of the two.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.