ID's are unique.
Each element can have only one ID.
Each page can have only one element with that ID.
Classes are NOT unique.
You can use the same class on multiple elements.
You can use multiple classes on the same element.
format the elements within their container so that they look correct regardless of the size of the container.
similarly format those containers within their own containers
<body> is the containers
never use absolute widths (px, in, cm, etc.) for anything
body tag takes up the whole width and height of the browser screen.
Pesticide Chrome Extension - Invaluable for debugging CSS.
CSS Blocks - High performance, maintainable stylesheets.
Opticss - CSS Optimizer.
Min - World's smallest (995 bytes) CSS framework.
Emotion - Performant and flexible CSS-in-JS library.
astroturf - An "artificial" css-in-js for those that want it all.
Pesticide - Kill your CSS layout bugs.
Spectre - Lightweight, Responsive and Modern CSS Framework.
Tachyons - Functional CSS for humans.
CSStype - Strict TypeScript and Flow types for style based on MDN data.
normalize.css - Modern alternative to CSS resets.
CSS Modules - Documentation about css-modules.
nano-css - CSS-in-JS library that you can actually use in production. Motto of nano-css is simple: create the smallest possible CSS-in-JS library and provide all features of any other library through addons.
Styled System - Responsive, theme-based style props for building design systems with React.
Rebass - React primitive UI components built with styled-system..
TypeStyle - Making CSS type safe.
CSS Scan Chrome Extension - Instantly inspect and copy computed CSS.
DropCSS - Simple, thorough and fast unused-CSS cleaner.
Water.css - Just-add-css collection of styles to make simple websites just a little nicer.
Loaders.css - Delightful and performance-focused pure css loading animations.
CSSFX - Beautifully simple click-to-copy CSS effects.
CSS-Only Chat - Truly monstrous async web chat using no JS whatsoever on the frontend.
Theme UI - Build consistent, themeable React UIs based on design system constraints and design tokens.
cssdb - Comprehensive list of CSS features and their positions in the process of becoming implemented web standards.