How to Identify and Reduce Unused CSS

27 Jun 2024 | 6 min read
How to Identify and Reduce Unused CSS

Understanding Unused CSS

Unused CSS refers to CSS code loaded on a webpage but not used to style any elements on that page. It often builds up over time as a website grows and evolves. Let’s break it down.

Unused CSS is like having clothes in your closet that you never wear. They take up space but don’t serve any purpose. Unused CSS can slow down website page load times because the browser has to download, parse, and process these extra styles.

Why Does Unused CSS Exist?

There are several reasons unused CSS accumulates on websites:

Legacy Code: Over time, websites get updated. Old styles may no longer be needed but often remain in the stylesheet.

Themes and Frameworks: Using themes or frameworks can add a lot of CSS, not all of which is necessary for every page on your site.

Third-Party Libraries: Adding libraries for buttons or forms often includes extra CSS you might not use.

Development Changes: As developers work on a site, they might create new styles and forget to remove the old ones.

How Does Unused CSS Impact Performance?

When a browser loads a webpage, it reads and processes all the CSS files. If these files contain a lot of unused CSS, it can:

Increase Load Time: More CSS means larger files, which take longer to download.

Delay Rendering: The browser takes time to apply styles. Unused CSS makes this process slower.

Affect User Experience: Slow-loading pages can frustrate users, leading to higher bounce rates.

Simple Example:

Imagine you have a CSS file with buttons, headers, and footer styles. If your page doesn’t have a footer, the footer styles are unused CSS. They still get loaded, but wait to do anything on that page.

Think of unused CSS, like packing unnecessary items for a trip. Carrying extra things you don’t need makes your bag heavier and more complex to manage. Similarly, unused CSS makes your webpage heavier and slower.

Identifying Unused CSS

Identifying unused CSS helps you make your website faster and more efficient. Let’s look at some simple methods for finding and removing this extra code.

Using Chrome DevTools

Chrome DevTools is a powerful tool that can help you identify unused CSS:

Open DevTools: Right-click on your webpage and select “Inspect” to open DevTools.

Go to the Coverage Tab: Click on the three dots in the top-right corner of DevTools, then navigate to “More Tools” and select “Coverage.”

Reload Your Page: Click the reload button at the top left of the Coverage tab. This will show how much of your CSS is used or unused.

The Coverage tab will display a list of CSS files with the percentage of used and unused code. This makes it easy to see which files need cleaning up.

Using Online Tools

Several online tools can help you find unused CSS:

PurifyCSS: This tool scans your HTML and JavaScript files to identify which CSS rules are used. It then removes the unused ones.

UnCSS: UnCSS works similarly by removing unused CSS rules from your stylesheets.

These tools can save time and ensure you get all the unused styles.

Manual Auditing

You can also manually audit your CSS, though it’s more time-consuming:

Review Your Stylesheets: Go through your CSS files and look for rules that aren’t applied to any elements on your site.

Check Your HTML: Compare your HTML and CSS to see which styles are being used.

While manual auditing can be thorough, it’s easy to overlook some unused styles.


Suppose your CSS file has a class .footer-link that styles links in the footer, but your current page doesn’t have a footer. This means the .footer-link class is unused CSS on that page.

Think of unused CSS like extra clutter in your room. Removing the clutter makes your space cleaner and easier to use. Similarly, removing unused CSS makes your website faster and more efficient.

Reducing Unused CSS

After identifying unused CSS, the next step is to reduce it. Here are some practical ways to clean up your CSS and improve your website’s performance.

CSS Audits

Perform regular CSS audits to keep your stylesheets clean and efficient.

Schedule Audits: Set a routine, like once a month, to review and clean your CSS.

Use Tools: Tools like PurifyCSS and UnCSS can help automate this process by scanning your files and removing unused styles.

Modular CSS

Write modular CSS to keep your code organized and reduce the risk of unused styles.

Create Components: Break your CSS into small, reusable components. For example, have separate files for buttons, headers, and footers.

Load Only What’s Needed: Only load the CSS components required for the current page. This keeps your CSS files smaller and more efficient.

Conditional Loading

Load CSS conditionally based on the user’s interaction or the page’s content.

JavaScript Loading: Use JavaScript to load CSS files only when needed. For instance, load additional styles when a user clicks a button or navigates to a new section.


<link rel="preload" href="styles.css" as="style" onload="this.onload=null;this.rel='stylesheet'">

<noscript><link rel="stylesheet" href="styles.css"></noscript>

Media Queries: Use media queries to load CSS for specific devices or screen sizes. This ensures that only the necessary styles are applied.

Minification and Compression

Reduce the size of your CSS files through minification and compression.

Minify CSS: Tools like CSSNano and CleanCSS can minify your CSS by removing whitespace, comments, and unnecessary characters.

Compress Files: Use gzip or Brotli to compress your CSS files. Most servers support these compression methods, which can significantly reduce file size and improve load times.

Critical CSS and Lazy Loading

Separate critical CSS from non-critical CSS to improve page load speed.

Inline Critical CSS: Inline the essential CSS directly in your HTML document. This ensures that the crucial styles load first. 



/* Critical CSS here */


Lazy Load Non-Critical CSS: Load the rest of your CSS asynchronously using JavaScript. This can be done after the main content has loaded so it doesn’t block rendering.


<link rel="preload" href="non-critical.css" as="style" onload="this.onload=null;this.rel='stylesheet'">

<noscript><link rel="stylesheet" href="non-critical.css"></noscript>


Imagine a webpage with a large CSS file that includes styles for multiple pages. By breaking this CSS into smaller, modular files and only loading what’s necessary, you can reduce the file size and improve load times.

Think of reducing unused CSS, like cleaning out your closet. Keeping only the clothes you wear makes it easier to find what you need and keep your space organized. Similarly, reducing unused CSS makes your website faster and more efficient.

Best Practices for Maintaining Clean CSS

Keeping your CSS clean and efficient is essential for a fast, well-functioning website. Here are some best practices for maintaining clean CSS.

Regular Reviews

Schedule Regular Checks: Set a routine to review your CSS files, like once a month.

Use Tools: Utilize tools like PurifyCSS or UnCSS to identify and remove unused CSS regularly.

Write Modular CSS

Break Into Components: Divide your CSS into small, reusable modules or components. For example, create separate files for buttons, headers, and footers.

Keep It Simple: Write simple, focused CSS rules for each element. This makes it easier to manage and update.

Avoid Overuse of Libraries

Use Only What You Need: Avoid loading entire CSS libraries if you only need a few styles. Extract and use only the necessary parts.

Custom Styles: Whenever possible, write custom styles instead of relying on large libraries. This reduces bloat and keeps your CSS lean.


Comment Your Code: Add comments to your CSS to explain the purpose of different styles. This helps others (and future you) understand the code.

Maintain a Style Guide: Create and maintain a style guide that outlines how to write and organize CSS for your project. This ensures consistency across your team.

Collaborative Development

Team Guidelines: Establish guidelines for your team on how to write and organize CSS. This helps prevent unused styles from accumulating.

Code Reviews: Implement code reviews for CSS changes. This ensures that new styles are necessary and follow your guidelines.


Suppose you have a CSS file with different buttons, headers, and footer styles. You keep your CSS organized and easily managed by breaking these into separate files and documenting each.

Maintaining clean CSS is like keeping a tidy desk. Regularly organizing and cleaning your desk makes it easier to find what you need and work efficiently. Similarly, maintaining clean CSS keeps your website running smoothly.

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