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CSS Handbook

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CSS Optimization Techniques

2 minute

CSS Optimization Techniques

A lean, efficient CSS setup directly impacts page speed, user experience, and SEO. Let’s break down the top optimization techniques.

Minify CSS

Minifying CSS is the process of stripping out all unnecessary characters — spaces, line breaks, and comments — without changing how your site looks. Think of it as packing your clothes neatly to fit more in a suitcase.

How to do it:

  • Use tools like cssnano, CleanCSS, or UglifyCSS.
  • Integrate minification into your build process with Webpack, Vite, or Parcel.
  • Keep only essential comments (like licensing) if needed.

Example:

<!-- index.html -->
<h1>Welcome</h1>
<p>This page loads quickly thanks to minified CSS!</p>

/* Before minification */
body {
  margin: 0;
  font-family: Arial, sans-serif;
  background-color: #ffffff;
}

h1 {
  color: #333;
  padding: 20px;
}

/* After minification */
body{margin:0;font-family:Arial,sans-serif;background-color:#fff}h1{color:#333;padding:20px}

Purge Unused CSS

Over time, CSS files accumulate rules that aren’t used on a page. Purging removes them, keeping your CSS light and clean.

How to do it:

  • Use PurgeCSS, UnCSS, or Tailwind Purge for framework projects.
  • Scan HTML and templates to detect unused styles.
  • Safelist dynamically added classes to avoid accidental removals.

Example (Tailwind CSS):

<h1 class="text-2xl font-bold">Hello, World!</h1>

/* Before purging */
.text-2xl { font-size: 1.5rem; }
.font-bold { font-weight: 700; }
.bg-blue { background-color: blue; } /* Unused */

/* After purging */
.text-2xl { font-size: 1.5rem; }
.font-bold { font-weight: 700; }

Command:

npx purgecss --css styles.css --content index.html --output optimized.css

Reduce Reflows and Repaints

When the browser recalculates layouts (reflow) or redraws pixels (repaint), it consumes processing power. Excessive reflows can slow your site and cause layout shifts.

How to do it:

  • Use transform and opacity for animations (GPU-accelerated).
  • Avoid changing layout properties like width, height, or margin repeatedly.
  • Batch DOM updates to reduce triggers.
  • Use will-change sparingly to hint at upcoming changes.

Example:

/* Bad: Causes reflow */
.btn:hover {
  width: 150px;
}

/* Good: Smooth, no reflow */
.btn:hover {
  transform: scale(1.1);
  background-color: navy;
}

<button class="btn">Hover Me</button>


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