SKILL.md

name	css-mixin
description	Pattern for mixins in native CSS (no preprocessor) — a mixin is a class in a shared CSS Module, its parameters are CSS Custom Properties, applied by adding the class to an element. Apply when creating or using reusable style mixins in a CSS Modules codebase.
license	MIT
compatibility	Claude Code Codex Cursor Gemini CLI GitHub Copilot Windsurf Cline Roo Code Goose Continue OpenCode Amp universal
metadata	author tags dangreen css css-modules mixins ui

CSS Mixin

In native CSS a mixin is a class:

• the mixin's parameters are CSS Custom Properties, with defaults declared on the class itself;
• to use a mixin, add its class to the element and override the CSS variables as needed.

Example

Mixins live in a shared file, e.g. mixins.module.css. A focusOutline mixin:

.focusOutline {
  --transitionFocusOutline: outline var(--durationSm) ease;
  --colorFocusOutline: var(--colorBrandLight);
  transition: var(--transitionFocusOutline);
  outline: var(--sizeBorderFocus) solid transparent;

  &:is(input, textarea):focus {
    outline-color: var(--colorFocusOutline);
  }

  &:not(:is(input, textarea)):focus-visible {
    outline-color: var(--colorFocusOutline);
  }
}

@media screen and (prefers-reduced-motion: reduce) {
  .focusOutline {
    transition: none;
  }
}

Usage:

import clsx from 'clsx'
import mixins from '~/uikit/mixins.module.css'
import styles from './Button.module.css'

export function Button({
  className,
  variant = 'primary',
  children,
  ...props
}) {
  return (
    <button
      className={clsx(className, styles.root, styles[variant], mixins.focusOutline)}
      {...props}
    >
      {children}
    </button>
  )
}