Optimize CSS stylesheets for your Svelte apps
Last updated 3 months ago by metonym .
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$ cnpm install caligula 
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Optimize CSS stylesheets for your Svelte apps.

This library statically analyzes and extracts class selectors from Svelte components using the svelte compiler. Given an external CSS file, the library outputs an optimized stylesheet by removing unused class rules.


One of the quickest ways to style Svelte applications (or web apps in general) is to define an external CSS stylesheet in the HTML head tag.

<!-- index.html -->
  <link rel="stylesheet" href="" />

By design, styles written in Svelte are scoped to the component. While scoped styles improve encapsulation, it is cumbersome to prefix globally applied rules with :global.

<!-- App.svelte -->
  import Accordion from "./Accordion.svelte";

  :global(.line-height-heading) {
    line-height: 1.2;

  :global(.font-size-sm) {
    font-size: 0.93rem;

<Accordion class="line-height-heading font-size-sm" />

The problem is that pre-built stylesheets contain far more rules than are actually used. The CSS file for a design system or library can be hundreds of kilobytes, even after minification.

For example, the stylesheet for the U.S. Web Design System weighs in at 268 kB minified.

Against Preprocessors

One solution is to use a preprocessor to import smaller SASS/SCSS partials.

This has two main drawbacks:

  • Extra build configuration: Using CSS preprocessors involves additional tooling and set-up. This is overkill, especially for rapid prototyping.
  • Still a manual process: SASS partials must be manually added or removed, which can be inefficient and error prone.

Getting Started

yarn add -D caligula


Install caligula as a development dependency.

yarn add -D caligula

In this example, only several class selectors are used from a localy copy of the U.S. Web Design System CSS file (unminified).

// postbuild.js
const caligula = require("caligula");

  include: ["src/**/*.svelte"],
  input: "css/uswds.css",


node postbuild.js

# Detected 7 classes from 2 Svelte components
# Removed 4660 classes from "css/uswds.css"
- Original size: 357.915 kB
+ New size: 10.53 kB

+ > 347.385 kB (97.1%) smaller!
+ > Saved output to "css/uswds.8a6dce134044.css"

The output file is minified and hashed. Its size is significantly smaller than that of the original.


   * glob of Svelte components
   * @type {Array.<string>}
  include: ["src/**/*.svelte"],

   * path to the original CSS file
   * @type {string}
  input: "css/uswds.css",

   * optional output file path
   * @type {string} [output=undefined]
  output: "dist/uswds.min.css",

   * hash the output file name
   * @type {boolean} [hash=true]
  hash: false,

   * cssnano minification config options
   * @type {object} [minifyOptions={ from: undefined }]
  minifyOptions: {},

   * hook triggered after minifying the CSS
   * useful for appending metadata like licenses
   * @param {string} - minified CSS
   * @returns {string} - modified CSS
  onMinify: (css) => `/*! uswds v2.6.0 */${css}`,



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