Core Layers

rules: [
  [/^m-(\d)$/, ([, d]) => ({ margin: `${d / 4}rem` }), { layer: 'utilities' }],
  ['btn', { padding: '4px' }], // default layer
]

layers: {
  'components': -1,
  'default': 1,
  'utilities': 2,
}

import 'uno:components.css'
import 'uno.css'
import './my-custom.css'
import 'uno:utilities.css'

outputToCssLayers: true

// Or with custom names
outputToCssLayers: {
  cssLayerName: (layer) => {
    if (layer === 'default') return 'utilities'
    if (layer === 'shortcuts') return 'utilities.shortcuts'
  }
}

<!-- UnoCSS layer -->
<p class="uno-layer-my-layer:text-xl">

<!-- CSS @layer -->
<p class="layer-my-layer:text-xl">

preflights: [
  {
    getCSS: ({ theme }) => `
      * {
        color: ${theme.colors.gray?.[700] ?? '#333'};
        margin: 0;
      }
    `,
  },
]

preflights: [
  {
    layer: 'base',
    getCSS: () => `html { font-family: system-ui; }`,
  },
]