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How CSS Modules scope styles
CSS Modules let you write ordinary CSS while your project’s build integration treats class selectors as local to the module by default. Importing a module gives you a mapping from the names you wrote to generated names used in the output. The CSS Modules project describes the compilation target as ICSS, a low-level interchange format. This approach is not limited to React; JSX is used here because it makes the mapping easy to see.
For example, a .button class in one module and a .button in another can coexist: each module’s local name is mapped independently. In component markup, refer to the class through the imported object rather than writing the generated name yourself. See the CSS Modules project documentation.
Define and use a local class
Create a module stylesheet and import it in the component or code that needs the styles.
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/* Card.module.css */
.card {
border: 1px solid #ddd;
}
.title {
font-weight: 700;
}
import styles from './Card.module.css';
export function Card() {
return (
<article className={styles.card}>
<h2 className={styles.title}>Title</h2>
</article>
);
}
styles.card and styles.title refer to the generated class mappings produced by the project’s CSS Modules integration. The exact generated spelling is an implementation detail and can vary; use the exported mapping instead of hard-coding it. Keep the CSS selector name and the mapping property aligned, and make sure the module is imported where its styles are used.
Use global selectors only as deliberate exceptions
When a style must target a global hook—such as a class supplied by a third-party library—CSS Modules documents :global(...) as an escape hatch. For example:
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/* Widget.module.css */
:global(.vendor-widget) {
margin-block: 1rem;
}
This explicitly targets the global .vendor-widget class instead of creating a local class mapping for it. Use global selectors for integration points, not as a substitute for local component styles. See the project’s CSS Modules documentation for its documented global-selector forms.
Combine local classes with composition
The composes declaration lets one local class include another local class, including one defined in a different module:
/* Button.module.css */
.base {
font: inherit;
border: 0;
}
.primary {
composes: base;
background: navy;
color: white;
}
The exported mapping for styles.primary includes the composed class as well as the class generated for primary. Composition has constraints: it applies to a single local class selector, and its declarations must appear before other declarations in that rule. Circular composition has undefined override behavior and may produce an error, so keep composition dependencies one-way. The CSS Modules composition documentation describes these rules.
Configure CSS Modules in a framework
CSS Modules behavior depends on the framework or build integration. In Next.js, the documented filename convention is .module.css, and importing the file yields a styles object. Follow the documentation for the router and framework version actually used by the project.
Next.js Pages Router
For the Pages Router, Next.js recommends importing site-wide global CSS at the application root. CSS import order can affect predictable production output, so keep global imports deliberate and consult the Pages Router CSS guide for the applicable version.
Next.js App Router
The App Router documentation permits global CSS imports in layouts, pages, or components and describes production CSS concatenation and code splitting. Do not apply the Pages Router placement rule as if it were universal; use the App Router CSS guide for the router in use.
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What local scope does—and does not—guarantee
CSS Modules prevent collisions between local class names through build-time mapping. They are not Shadow DOM, a runtime security boundary, or complete isolation from the cascade. Global selectors and element selectors still participate in ordinary CSS; inherited properties, custom properties, specificity, and import order can also affect rendered styles. Treat the module boundary as a way to manage class selector names, not as a promise that no outside CSS can affect an element.
Common problems and fixes
- The class is missing or styles do not apply: Confirm the stylesheet follows the module convention recognized by your framework (in Next.js,
.module.css), is imported, and the markup uses the exported mapping such asstyles.card. - A global or vendor class is not targeted: A module’s ordinary class selectors are local. Use the documented
:global(...)form for a deliberate global selector. - The same class name appears in multiple modules: That is expected; local names are mapped per module. Do not replace mapping references with guessed generated names.
- Composed styles fail or have surprising precedence: Check that
composesapplies to one local class selector, appears before other declarations in the rule, and does not form a circular dependency. - Global CSS behaves differently in production: Review import placement and ordering for your framework and router. Next.js documents different global CSS guidance for its Pages and App Routers.
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