Assembly

A CSS framework that makes the hard parts of building anything on the web easy. We define the hard parts as: managing class specificity, designing cross-browser form components that work well with each other, creating a harmonious typographic scale, maintaining a baseline grid, and keeping responsive designs simple.

For usage guidelines and documentation, check out https://labs.mapbox.com/assembly/.

Browser support

Assembly targets modern versions of Chrome, Firefox, Safari, and Edge on desktop; Safari on iOS; and Chrome on Android.

Usage

See https://labs.mapbox.com/assembly/ for usage guidelines.

Migration

See our Migration guides to understand what changes you might need to make when upgrading Assembly in your project.

Custom Builds

Assembly exposes a JS module for creating custom builds. Why might you want to create a custom build?

• You want to customize variables (like colors and font stacks) or media queries.
• You want to append extra stylesheets that also use Assembly's variables and media queries.
• You want to reduce file-size by picking and choosing the color variants you need.

presetAssembly([options])

Returns an UnoCSS preset that generates Assembly class names on demand. Utilities, media variants, dark: variants, and color modifiers are tree-shaken from your source. Reset, form, and other component CSS is included as preflight. See Dark mode for the dark: prefix.

import { defineConfig } from 'unocss';
import { presetAssembly } from '@mapbox/assembly';

export default defineConfig({
  presets: [presetAssembly()]
});

The generated CSS uses Assembly's CSS variables (var(--blue)) and custom media queries (@media (--m-screen)), which browsers can't resolve on their own. Run it through PostCSS with the same plugins buildUserAssets uses, pointed at Assembly's variable and media query definitions:

// postcss.config.js
const variables = require('@mapbox/assembly/src/variables.json');
const mediaQueries = require('@mapbox/assembly/src/media-queries.json');

module.exports = {
  plugins: [
    require('postcss-custom-properties')({
      preserve: false,
      importFrom: { customProperties: variables }
    }),
    require('postcss-custom-media')({
      importFrom: { customMedia: mediaQueries }
    })
  ]
};

importFrom requires postcss-custom-properties@12 and postcss-custom-media@8; later major versions removed it. To change a variable or breakpoint, override its entry in these objects. examples/jit/build.mjs shows the full pipeline.

The prebuilt assembly.css is generated with presetAssembly({ safelist: true }) so every class is present. Do not pass safelist: true in an app that wants on-demand CSS.

options.colorVariants and options.files match the buildUserAssets options of the same names. Variable overrides are applied in the PostCSS step above, not by the preset.

buildUserAssets(outdir[, options])

Returns a Promise that resolves when all assets have been written to outdir.

const Assembly = require('assembly');
Assembly.buildUserAssets('path/to/my/outdir', myOptions)
  .then(() => /* something */)
  .catch((err) => /* handle error */);

Options, all of which are optional:

• files: An array of file paths to stylesheets you would like to append to assembly.css. These will be processed through Assembly's PostCSS pipeline.

• variables: An object whose properties will override and add to src/variables.json. Use this option to change or add variables. These variables are accessible in any stylesheets you append via the CSS custom properties syntax, e.g. var(--property-name).

• mediaQueries: An object whose properties will override and add to src/media-queries.json. Use this option to change or add media queries. These media queries are accessible in any stylesheets you append via the CSS custom media query syntax, e.g. @media --media-query-name.

• colorVariants: An object or array specifying the color variants you would like added to assembly.css. This is documented in detail below.

• icons: An array of icons names to include in Assembly. Names correspond to file names in src/svgs/. Use this option to decrease the size of assembly.js by only including the icons you need.

• browsersList: A Browserslist array to be used by Autoprefixer. Default:

  [
    'last 4 Chrome versions',
    'last 4 Firefox versions',
    'last 4 Safari versions',
    'last 2 Edge versions',
    'last 2 iOS versions',
    'last 2 Android versions',
    'not IE 11',
    'not dead'
  ]
  

• quiet: Suppress logs.

colorVariants option

If the colorVariants value is an array, it must be an array of color names corresponding to variables. All components will have color variants generated for all colors in the array.

The following configuration specifies an array of default colors. All components will have these (and only these) color variants.

["red", "green-light"]

If the colorVariants value is an object, each property value must be an array of color names corresponding to variables. The property names designate which component each color array applies to:

• default: These colors apply to all components that are not otherwise specified.
• buttonFill: *-dark colors will not be used.
• buttonStroke: *-dark colors will not be used.
• inputTextarea: *-dark colors will not be used.
• selectFill: *-dark colors will not be used.
• selectStroke: *-dark colors will not be used.
• checkbox: *-dark colors will not be used.
• radio: *-dark colors will not be used.
• switch: *-dark colors will not be used.
• toggle: *-dark colors will not be used.
• range: *-dark colors will not be used.
• color
• background
• link: *-dark colors will not be used.
• border
• hoverShadow: Only lighten* and darken* colors will be used.
• hoverBackground
• hoverColor
• hoverBorder

The following configuration specifies colors for individual components. In this configuration, every component not specified will have the default color variants; specified components will have their specified color variants; and switch and range components will have no color variants (only the default will be available).

{
  "default": ["lighten50", "lighten25", "gray"],
  "buttonFill": ["green", "purple"],
  "selectFill": ["green"],
  "background": ["orange", "yellow", "pink"],
  "link": ["orange"],
  "hoverShadow": ["lighten50"],
  "switch": [],
  "range": []
}

Development principles

Writing rules

Assembly strives for flat, single rule declarations and avoids overrides whenever possible. Declarations are marked as !important for classes whose names directly correspond with what they do — e.g. bg-blue, px12. This way they cannot be overridden and so are guaranteed to actually do what they say they do.

Naming classes

• Keep names as short as reasonable.
• Use real number values in utility class names to describe the value the utility class applies in cases where the number of utility classes describing a particular property could be unlimited. For example, .pt6 for padding-top: 6px instead an abstract scale like .pt-small or .pt-1.
• If the number of utility classes describing a property is limited and the variants are about size, Assembly classes use the suffixes xl, l, m, s, xs.
• Assembly provides a reset that will affect the entire page, but other than that reset none of its rules should affect the styling of elements that don't bear Assembly classes.

Media query class variants

Media query class variants (e.g. block-mm as a variant of block) are generated by the Assembly UnoCSS preset (src/preset/variants.js). The prebuilt stylesheet includes -mm, -ml, and -mxl variants for the classes listed in src/preset/media-classes.js plus every layout-scale utility. If you use presetAssembly() on demand, those suffixes work for any matching utility.

Dark mode

With presetAssembly() on demand, prefix any Assembly class with dark: to apply it only inside @media (prefers-color-scheme: dark). The prebuilt assembly.css doesn't include dark: classes. Assembly has no dark palette of its own, so pair the classes you already use:

<body class="bg-white dark:bg-gray-dark color-gray-dark dark:color-white">
  <button class="btn btn--blue dark:btn--green">Save</button>
  <div class="loading dark:loading--dark"></div>
</body>

• dark: works with every class, including component classes such as btn, loading, table--dark, and prose--dark, along with their hover and active states.
• It combines with media query suffixes: dark:block-mm applies block at the --m-screen breakpoint in dark mode.
• A dark: class follows its light and breakpoint counterparts in the output, so with block-mm dark:none the element is hidden in dark mode at every width. Write dark:block-mm to keep it visible from that breakpoint up.

Development

Tools

• UnoCSS generates Assembly utilities from presetAssembly() (src/preset/utilities.js, layout scales, color rules, media-query variants, and src/preset/base.css for reset/forms).
• PostCSS for processing CSS. PostCSS parses CSS and runs it through plugins, and these are the plugins we're using:
  • Autoprefixer automatically adds vendor prefixes.
  • postcss-custom-properties allows us to use variables for values, with the CSS custom properties syntax.
  • postcss-custom-media allows us to use variables for media queries, with the CSS custom media queries syntax.
• SVGs
  • svgstore compiles our SVGs into a SVG "sprite" of sorts, allowing us to use the latest and greatest SVG-based icon system.
• VitePress powers the website.

Install and start

npm ci # Installs your `node_modules`

npm run dev # Builds CSS/JS, starts the VitePress docs server

npm run build:js # Build SVGs and other JS

For other scripts, look in package.json.

Releasing

Development is done in the main branch, but releases are made from the publisher-production branch (it will go away once the old Publisher-triggered deploy is retired). Here's how you cut a release:

• From main:
  • Document changes in the CHANGELOG.
  • Increment the version key in package.json and package-lock.json.
  • Make sure all this is committed, typically with a commit message like Prepare 0.8.0.
  • Merge these changes into the publisher-production branch. Conduct the following steps from publisher-production.
• From publisher-production:
  • Create a tag matching the version, prefixed with v. For example: git tag v0.8.0.
  • Push the tag: git push origin v0.8.0.
  • This triggers the NPM release GitHub Actions workflow, which publishes to npm via Trusted Publishing (OIDC) — no npm token needed.
• Any prerelease tag (a version containing -, e.g. v0.8.0-dev.0) publishes under the npm dev dist-tag instead of latest, and can be cut from any branch for testing.
• Non-dev tags must point to a commit on publisher-production, or the workflow fails fast.