CSS, Sass, and Less in browser extensions

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Template examples

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content-sass

npx extension@latest create my-extension --template=content-sass

pnpx extension@latest create my-extension --template=content-sass

yarn dlx extension@latest create my-extension --template=content-sass

bunx extension@latest create my-extension --template=content-sass

deno run -A npm:extension@latest create my-extension --template=content-sass

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content-custom-font

npx extension@latest create my-extension --template=content-custom-font

pnpx extension@latest create my-extension --template=content-custom-font

yarn dlx extension@latest create my-extension --template=content-custom-font

bunx extension@latest create my-extension --template=content-custom-font

deno run -A npm:extension@latest create my-extension --template=content-custom-font

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CSS capabilities

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Where to reference CSS

• manifest.json (content_scripts[].css)
• HTML files (<link rel="stylesheet" href="...">)
• Script imports (import "./styles.css", including Sass/Less when enabled)

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CSS support

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Example: CSS in manifest.json

{
  "manifest_version": 3,
  "name": "My Extension",
  "version": "1.0.0",
  "content_scripts": [
    {
      "matches": ["<all_urls>"],
      "css": ["./styles/content.css"],
      "js": ["./scripts/content.ts"]
    }
  ]
}

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Example: CSS in extension page scripts

import "./styles/popup.scss";

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Output behavior by context

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Development behavior

• Content script CSS imports participate in the content-script HMR/remount flow.
• Extension.js adds a dev helper script to CSS-only content script entries so it can propagate updates.
• Page CSS follows normal page HMR pipeline behavior.
• Structural manifest/content-script changes can still require full extension reload or restart.

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Modules and preprocessors

• CSS modules work best in extension page contexts.
• Extension.js enables Sass/Less support when you install the related dependencies.
• If a project references .scss/.less files but the preprocessor is not installed, the build warns and ships the stylesheet as plain CSS instead of failing, matching how the browser loads whatever CSS it receives. Install the preprocessor to get real compilation.
• Extension.js runs PostCSS automatically when it detects a PostCSS configuration in your project, or when you configure it explicitly.

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Best practices

• Keep content-script styles intentionally scoped to reduce host-page collisions.
• Prefer component-local module styles for extension page UIs.
• Keep preprocessor and PostCSS configurations explicit to avoid unintended changes to your build setup over time.
• Validate CSS paths referenced by manifest fields in continuous integration (CI).

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Next steps

• Understand update outcomes in dev update behavior.
• Learn more about CSS modules.
• Learn more about PostCSS integration.