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How to Remove Unused CSS and Make Your Website Load Faster

Optimizing website CSS code

Short answer: unused CSS bloats your stylesheet and slows page loads. To fix it, find what is actually used with Chrome DevTools' Coverage tool, then remove the dead rules manually or automatically with a tool like PurgeCSS, and always test afterward. Here is the safe process, especially important if you use a big framework like Bootstrap.

Why unused CSS matters

Frameworks and themes ship huge stylesheets, but a typical page uses only a fraction of the rules. The browser still downloads and parses all of it, delaying rendering. Trimming unused CSS shrinks the file and speeds up load, a real Core Web Vitals win.

Step 1: See how much CSS is unused

  1. Open your page in Chrome and press F12 for DevTools.
  2. Open the Coverage tab (Ctrl+Shift+P, type "Coverage").
  3. Click reload; it shows each CSS file with the percentage of unused bytes, often shockingly high.

This tells you how much you can save and which files to target.

Step 2: Remove the unused rules

  • Automatically: PurgeCSS scans your HTML/JS and strips CSS selectors you never use. It integrates into most build tools.
  • Manually: for a small site, delete obviously unused blocks and framework components you do not use.
  • Build tools: modern setups (Tailwind, Vite, etc.) purge unused CSS automatically in production.

Step 3: Test thoroughly (the critical step)

Removing CSS can break styling that only appears in certain states, hover effects, modals, error messages, mobile menus. After purging:

  • Click through every interactive element and state.
  • Test on mobile and desktop widths.
  • Check dynamically added content (JS-generated elements).

The non-obvious tip: watch out for dynamic class names

Automated purgers can wrongly delete CSS for classes that only get added by JavaScript at runtime (like an "is-open" class), since they are not in the static HTML. Configure a safelist for those classes, or you will get mysterious broken styling that only shows up during interaction. This is the number-one cause of "PurgeCSS broke my site", and it is easily avoided by safelisting dynamic classes.

Frequently asked questions

How do I find unused CSS?

Use Chrome DevTools' Coverage tab (Ctrl+Shift+P, type Coverage). Reload the page and it shows the percentage of unused CSS in each file.

How do I remove unused CSS automatically?

Use PurgeCSS to scan your HTML and JS and strip unused selectors, or rely on modern build tools (Tailwind, Vite) that purge in production.

Does removing unused CSS speed up a website?

Yes. A smaller stylesheet downloads and parses faster, improving load time and Core Web Vitals, especially with large frameworks.

Why did removing CSS break my site's styling?

Automated tools can delete classes added by JavaScript at runtime. Add those dynamic class names to a safelist so they are not purged.

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