CSS Modules

CSS Modules — Opentrons Conventions

File Naming

File names are lowercase, no separators, suffixed with .module.css, and match the component name:

ComponentName/
├── index.tsx
├── componentname.module.css
└── **tests**/
└── ComponentName.test.tsx

Examples: navbar.module.css, labwarebutton.module.css, textareafield.module.css

Class Naming

Stylelint enforces the pattern /^[a-z0-9_]+$/ — snake_case only (lowercase letters, digits, underscores).

Use a component_element or element_modifier structure:

/* Base classes */
.button {
}
.slider_container {
}
.crumb_link {
}

/* State/modifier variants */
.button_active {
}
.textarea_error {
}
.title_text_center {
}
.crumb_link_inactive {
}

Importing in Components

import styles from './componentname.module.css'

export function MyComponent(): JSX.Element {
  return <div className={styles.wrapper}>content</div>
}

Dynamic Styling with clsx

Use clsx for combining or conditionally applying classes:

import clsx from 'clsx'

import styles from './componentname.module.css'

export function MyButton({ isActive, isError }: Props): JSX.Element {
  const className = clsx(styles.button, {
    [styles.button_active]: isActive,
    [styles.button_error]: isError,
  })
  return <button className={className}>Click</button>
}

Design Tokens (CSS Custom Properties)

All tokens are defined in components/src/styles/global.css. Always use these variables instead of hard-coding values for colors, spacing, typography, and border-radius.

Spacing

Colors

Core: --white (#fff), --black-90 (#16212d), --black-80 (#24313f), --black-70 (#39495b)

Grey: --grey-10 through --grey-60 (10, 20, 30, 35, 40, 50, 55, 60)

Blue: --blue-10 through --blue-60 (10, 20, 30, 35, 40, 50, 55, 60)

Purple: --purple-20 through --purple-60 (20, 30, 35, 40, 50, 55, 60)

Green: --green-20 through --green-60 (20, 30, 35, 40, 50, 60)

Red: --red-20 through --red-60 (20, 30, 35, 40, 50, 55, 60)

Yellow: --yellow-20 through --yellow-60 (20, 30, 35, 40, 50, 60)

Flex brand: --flex-40, --flex-50, --flex-55

Semi-transparent: --transparent-white-80, --transparent-white-50, --transparent-black-10, --transparent-black-60, --transparent-black-80, --transparent

Some colors (green, purple) have touchscreen variants that activate when components/src/styles/global.css applies the .enable_touchscreen_colors class. The default values are non-touchscreen.

Border Radius

--border-radius-2, --border-radius-4, --border-radius-8, --border-radius-12, --border-radius-16, --border-radius-40, --border-radius-200 (pill/circle)

Typography

Font size: --font-size-caption (10px), --font-size-label (11px), --font-size-p (13px), --font-size-h4 (13px), --font-size-h3 (14px), --font-size-h2 (15px), --font-size-h1 (19px), --font-size-20, --font-size-22, --font-size-28, --font-size-32, --font-size-38, --font-size-80

Font weight: --font-weight-light (300), --font-weight-regular (400), --font-weight-semi-bold (600), --font-weight-bold (700)

Line height: --line-height-12, --line-height-16, --line-height-18, --line-height-20, --line-height-24, --line-height-28, --line-height-36, --line-height-42, --line-height-48, --line-height-96

Where to Use Tokens vs Explicit Values

Unit Rules

• rem for all dimensions — divide design-spec px by 16 (240px / 16 = 15rem)
• px only for box-shadow values and <img> tag dimensions
• Prefer padding over margin for wrapper and container elements
• Avoid complex calc() expressions

State & Pseudo-Class Handling

.button:focus {
  outline: none;
  box-shadow: 0 0 0 2px var(--blue-50);
}

.button:hover:not(:disabled) {
  background-color: var(--grey-10);
}

.button:disabled {
  opacity: 0.5;
  cursor: not-allowed;
}

.input:focus {
  border-color: var(--blue-50);
}

Always guard hover styles with :not(:disabled) on interactive elements.

CSS Modules Features

Composition

Stylelint allows the composes property for class composition:

.base_button {
  padding: var(--spacing-8) var(--spacing-16);
  border-radius: var(--border-radius-4);
}

.primary_button {
  composes: base_button;
  background-color: var(--blue-50);
  color: var(--white);
}

Global Selectors

Use :global() to target non-module classes (e.g., from React Router or third-party libraries):

.nav_link:global(.active) {
  color: var(--blue-50);
}

Stylelint Configuration

The repo uses stylelint-config-standard + stylelint-config-idiomatic-order (property ordering). Key enforced rules:

• Class selector pattern: /^[a-z0-9_]+$/
• Max 4 decimal places for numbers (except dimension properties like width, height, flex)
• Standard CSS property ordering (idiomatic order)
• @value at-rule is still allowed but deprecated — avoid in new code

Linting and Formatting

# Lint all CSS
make lint-css

# Auto-fix all CSS
make format-css

# Lint a single file
pnpm stylelint path/to/componentname.module.css

# Auto-fix a single file
pnpm stylelint path/to/componentname.module.css --fix

Always run make lint-css and fix any issues before committing.

Legacy Patterns (Do Not Introduce)

• components/src/styles/borders.module.css — old border tokens (--bd-radius-*, --bd-width-*, --shadow-lvl-*). Use global.css tokens instead.
• components/src/styles/typography.module.css — old typography tokens (--fs-*, --fw-*, --lh-*). Use global.css tokens instead.
• styled-components — the codebase is migrating away. Always use CSS Modules for new styles.
• @value declarations — still parsed by stylelint but deprecated. Use CSS custom properties.