core-js is the more common of the two in our index: it runs on 20,027 websites against 164 for Keen-Slider, and 80 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.
Keen-Slider is a free library agnostic touch slider with native touch/swipe behavior.
80 sites run both core-js and Keen-Slider. That is 49% of the sites using Keen-Slider, 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.