Codepen.io is predominantly used as a place for web designers to showcase their web designs, styles and ideas. You can easily search for assets or designs and see the source code (Html, CSS and JS). If you needed some ideas of buttons or forms you could search these up on Codepen.
Thats what im getting at, Codepen is an excellent source of inspiration and ideas. You don’t steal the designs and code from Codepen to be exact rather you see the methods and classes used to create that style.
Codepen is purely for front end development, ie code that can be seen or viewed using browser dev tools and no code like PHP. Most recently has been a place for a lot of JS and SVG animation showcases. Whilst these design or most of them are nice to look at you can’t really take a whole lot from this unless you’re into making web animations, i am not.
Codepen is costly
One gripe I have on Codepen is it is very expensive, $9 per month for the pro plan which gives you unlimited “pens” and private “pens” too. The Codepen free plan allows you 1 pen. Pretty useless really, which goes to say why im making the point to get ideas from Codepen rather than use it.