Usually this will give you a .min.js file ready for you to script include on your site, for example, unpkg.com/mithril
Sometimes library authors don't default the export to a browser ready minified version or ESM module, in which case, you can snoop around the built package at unpkg.com/browse/mithril/.
Most READMEs worth a damn should tell you how to use their libraries without npm if possible. Unfortunately, a lot of library authors suck.