It wasn't that long ago where you couldn't programmatically copy text to the clipboard from the web without using Flash. But it's getting pretty well supported these days. IE 10+, Chrome 43+, Firefox 41+, and Opera 29+, says Matt Gaunt in writing about it on Google's developer site.
That article says it's not supported in Safari, but it is as of Safari 10, which only dropped in September.
You don't need a library to do this stuff, as evidenced by the above demo. But, there is one: clipboard.js. It's only 3kb gzipped. The purpose is to give you a bit of a cleaner API to work with, like success and error events, and configuration through data-* attributes. Here's a demo of that: