Paged.js is a free and open source JavaScript library that paginates content in the browser to create PDF output from any HTML content. This means you can design works for print (eg. books) using HTML and CSS!

WebTUI is a modular CSS library that brings the beauty of Terminal UIs to the browser.

Project Wallace is a set of CSS analyzers that check your complexity, specificity, performance, Design Tokens and much more. And all of that in a single web app.