Unused CSS Rules: Cleaning Out the Style Closet You're Not Wearing

Where Do All These Unused Rules Come From?

Unused CSS doesn't appear by accident. Several common development practices contribute to CSS bloat:

• CSS Frameworks: Frameworks like Bootstrap, Foundation, or Bulma provide comprehensive styling systems, but most websites only use a fraction of their features.
• Theme Customization: Pre-built themes often include styles for dozens of layout variations and components you might never use.
• Historical Accumulation: As websites evolve, old styles for removed elements often remain in the stylesheets.
• Design Changes: When redesigning sections of a site, developers may add new styles without removing the old ones.
• Plugin and Widget Styles: Each plugin or third-party widget might add its own CSS, including rules for features you're not using.

The result is CSS bloat—stylesheets that are far larger than they need to be, containing rules that never apply to any element on your actual pages.

Why Unused CSS Creates Performance Problems

CSS is a render-blocking resource, which means browsers won't display your page until they've downloaded, parsed, and applied all CSS files. This makes CSS optimization particularly important for several reasons:

• Delayed Rendering: Larger CSS files take longer to download and process, directly delaying when users first see your content.
• Increased Memory Usage: The browser must store the entire CSS object model in memory, even for rules that are never used.
• Slower Style Calculations: When the browser updates the layout, it potentially needs to check more rules against each element.
• Mobile Impact: These issues are magnified on mobile devices with less processing power and often slower connections.

The scale of the problem can be surprising. Many websites, especially those using popular CSS frameworks or content management systems, have stylesheets where 50-90% of the rules are never used. That's an enormous amount of wasted bandwidth and processing time.

How to Identify Unused CSS on Your Website

Before optimizing, you need to understand the scope of the problem. Here are several approaches to identify unused CSS:

• Browser Developer Tools: Most modern browsers include coverage analysis in their developer tools, which can show you exactly which CSS rules are used and unused during your browsing session.
• Performance Audit Tools: Various performance testing tools can identify unused CSS as part of their analysis.
• CSS Analysis Libraries: Tools like PurgeCSS or UnCSS can analyze your site and generate reports of unused CSS.

When analyzing, it's important to:

• Check multiple pages on your site, not just the homepage
• Interact with elements that might trigger state changes (like hovering over menus)
• Test user flows that might load CSS conditionally

This comprehensive analysis will give you a clear picture of which CSS rules are actually being used across your website.

7 Effective Methods to Eliminate Unused CSS

// Example with Webpack and PurgeCSS
const purgecss = require('@fullhuman/postcss-purgecss');

module.exports = {
  plugins: [
    // Other plugins...
    purgecss({
      content: ['./src/**/*.html', './src/**/*.js'],  // Files to analyze
      defaultExtractor: content => content.match(/[w-/:]+(?<!:)/g) || []
    })
  ]
}

// Example with Gulp and UnCSS
const gulp = require('gulp');
const uncss = require('gulp-uncss');

gulp.task('css', function() {
  return gulp.src('css/styles.css')
    .pipe(uncss({
      html: ['index.html', 'about.html', 'blog/**/*.html']
    }))
    .pipe(gulp.dest('./dist/css'));
});

// Instead of importing all of Bootstrap
// import 'bootstrap/dist/css/bootstrap.min.css';

// Import only what you need
@import "~bootstrap/scss/functions";
@import "~bootstrap/scss/variables";
@import "~bootstrap/scss/mixins";
@import "~bootstrap/scss/grid";
@import "~bootstrap/scss/buttons";
// Only import the components you actually use

// Example with Critical
const critical = require('critical');

critical.generate({
  base: 'dist/',
  src: 'index.html',
  dest: 'index-critical.html',
  inline: true,
  dimensions: [
    {
      width: 320,
      height: 480
    },
    {
      width: 1200,
      height: 900
    }
  ]
});

// Example of CSS Modules in React
import styles from './Button.module.css';

function Button() {
  return <button className={styles.button}>Click Me</button>;
}

// This ensures only the CSS actually used by components gets included

Special Considerations and Potential Pitfalls

When removing unused CSS, be aware of these important considerations:

// PurgeCSS configuration with safelist
module.exports = {
  // ...
  safelist: [
    'active',
    'show',
    'hidden',
    /^modal-/,  // Preserve all classes starting with 'modal-'
    /^alert-/   // Preserve all classes starting with 'alert-'
  ]
}

The Performance Impact of Removing Unused CSS

Removing unused CSS can have a dramatic impact on your website's performance:

The benefits are especially significant for mobile users, where both bandwidth and processing power are more limited. By removing unused CSS, you're not just reducing download size—you're also reducing the amount of code the browser needs to parse, store in memory, and evaluate during style calculations.

Real-World Success Stories

Organizations across industries have seen remarkable improvements from optimizing their CSS:

• E-commerce platform implemented PurgeCSS and reduced their CSS from 380KB to 85KB (78% reduction). This contributed to a 1.2-second improvement in First Contentful Paint and a 15% increase in conversion rate, particularly on mobile devices.
• News website with a complex layout switched to a critical CSS approach and removed unused styles, cutting their render-blocking CSS by 83%. This helped improve their Core Web Vitals scores and increased ad viewability by 23% due to faster initial rendering.
• Corporate website built with a popular CSS framework eliminated unused components, reducing stylesheet size by 76%. This improved mobile page speed scores by 32 points and decreased bounce rates by 17%, especially for visitors from search engines.
• SaaS application transitioned to a CSS-in-JS approach with automatic pruning, ensuring only the styles needed for each view were delivered. This made their application feel more responsive on lower-end devices, improving user satisfaction metrics by 19%.

These examples demonstrate that CSS optimization can have a direct impact on the metrics that matter most to businesses: engagement, conversion, and user satisfaction.