What is UnoCSS?

UnoCSS is the instant atomic CSS engine, that is designed to be flexible and extensible. The core is un-opinionated and all the CSS utilities are provided via presets.

For example, you could define your custom CSS utilities, by providing rules in your local config file.

// uno.config.ts
import { defineConfig } from 'unocss'

export default defineConfig({
  rules: [
    ['m-1', { margin: '1px' }],
  ],
})

This will add a new CSS utility m-1 to your project. Since UnoCSS is on-demand, it won't do anything until you use it in your codebase. So say we have a component like this:

html

<div class="m-1">Hello</div>

m-1 will be detected and the following CSS will be generated:

css

.m-1 { margin: 1px; }

To make it more flexible, you can make your rule dynamic by changing the first argument on the rule (we call it matcher) to a RegExp, and the body to a function, for example:

diff

// uno.config.ts
export default defineConfig({
  rules: [
-    ['m-1', { margin: '1px' }],
+    [/^m-([\.\d]+)$/, ([_, num]) => ({ margin: `${num}px` })],
  ],
})

By doing this, now you can have arbitrary margin utilities, like m-1, m-100 or m-52.43. And again, UnoCSS only generates them whenever you use them.

html

<div class="m-1">Hello</div>
<div class="m-7.5">World</div>

css

.m-1 { margin: 1px; }
.m-7.5 { margin: 7.5px; }

Presets

Once you made a few rules, you can extract them into a preset, and share it with others. For example, you can create a preset for your company's design system, and share it with your team.

// my-preset.ts
import { Preset } from 'unocss'

export const myPreset: Preset = {
  name: 'my-preset',
  rules: [
    [/^m-([\.\d]+)$/, ([_, num]) => ({ margin: `${num}px` })],
    [/^p-([\.\d]+)$/, ([_, num]) => ({ padding: `${num}px` })],
  ],
  variants: [/* ... */],
  shortcuts: [/* ... */],
  // ...
}

// uno.config.ts
import { defineConfig } from 'unocss'
import { myPreset } from './my-preset'

export default defineConfig({
  presets: [
    myPreset, // your own preset
  ],
})