An HTML and CSS only tree.
The final CSS-snippet is also available at the end.

It uses only <ul>, <li>,

Micro CSS util class for applying inflated fluffy 3D claymorphism styles to elements. Fully customizable and extensible with CSS variables. SASS mixin is also included for even more styling options.

It provides examples: https://siimple.xyz/examples

Also available on NPM for a 7.8kB minified and gzipped package.

Reasonable Colors is an open-source color system for building accessible, nice-looking color palettes.

Build HTML elements visually

Source code: https://github.com/components-ai/css.gui

Paper Prototype CSS is CSS framework that mimics paper prototyping, sort of like a handdrawn look but a little more ransom-note-ish, with the help of two informal-looking free fonts.

And the blog post related: https://shkspr.mobi/blog/2022/06/paper-prototype-css/

Metallic look to HML elements.

Source code: https://github.com/MikaeI/metallicss (available as npm package)

Generate animations with the mouse :D

Github: https://github.com/panphora/animatize

A bootstrap similar library

Copy/Paste the most used media query

Manual-writing styled forms with a CSS only file.

The font Short Stack should also be used. It is available as NPM package.

Another drop-in css stylesheet which make a website beautiful :)

Demo: https://alvaromontoro.github.io/almond.css/demo/

Using CSS to get a fingerprint of the browser. Here is a all-in-one resource.

#BADA55, #101 (lol), #C55 (CSS), #fb1 (FBI), etc...

It uses the l33t code for that.

It generates different layouts

Another collection of CSS loaders and spinners

Another list of CSS spinners

Customizable CSS loaders and spinners