picocss/pico

Pico CSS is a minimal CSS framework (a framework is a pre-built set of styling rules you add to your project) for semantic HTML. "Semantic HTML" means using standard HTML tags like headings, paragraphs, and forms in their intended way, and Pico makes those standard tags look clean and polished with almost no extra work. The idea is simple: you write plain HTML, link in one Pico CSS file, and your page immediately gets responsive layouts (meaning it adapts to any screen size), light and dark mode support, styled forms and buttons, and a consistent look, all without having to add CSS class names to every element. There is even a "classless" version where you just write pure HTML and Pico handles the rest. Version 2 added a complete color palette, over 100 precompiled color themes, and a new group component. Customization is available through CSS Variables (settings built into the browser) or SASS (a tool that extends CSS with extra features like variables and imports), but those are optional, the default setup works out of the box. Pico is best suited for quick prototypes, small projects, or as a clean starting point before you add your own styles. It is not a full utility toolkit, so larger projects may need additional CSS knowledge to extend it. The project is written in CSS, licensed under MIT (free to use for anything), and has over 16,000 stars on GitHub.