core-js is the more common of the two in our index: it runs on 17,397 websites against 29 for waitForImages, and 14 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.
waitForImages is a lightweight, high-performance JavaScript library that simplifies the handling of image preloading events.
14 sites run both core-js and waitForImages. That is 48% of the sites using waitForImages, 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.