Optimizing Website Css For Speed

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Implementing Reduce Unused CSS and Preloading CSS: Enhancing Website Performance.

In the digital age, website performance is not just a metric; it's a critical component of user experience and search engine optimization. Two effective techniques to boost your website's speed and efficiency are reducing unused CSS and preloading critical CSS. These strategies not only improve loading times but also contribute to a smoother user interaction. In this article, we'll explore how these methods can be implemented and their benefits, complete with examples.

Understanding the Basics

Unused CSS

Over time, websites evolve. New pages are added, designs change, but often, the CSS (Cascading Style Sheets) that styles these pages does not get cleaned up. This results in a lot of unused CSS rules that browsers still need to download, parse, and process, wasting valuable time and resources.

Preloading CSS

Preloading CSS is a technique to instruct the browser to load CSS files that are needed for above-the-fold content (the part of the webpage visible without scrolling) as soon as possible. This ensures that the critical styling is applied immediately as the page loads, improving perceived performance.

Implementing Reduce Unused CSS

The first step in reducing unused CSS is identifying it. Tools like Google's PageSpeed Insights and coverage tab in Chrome DevTools can help you spot CSS rules that are not being used by your website.

Once identified, you can take several approaches to reduce unused CSS:

  1. Manual Cleanup: Go through your CSS files and manually remove unused styles. This is feasible for smaller projects but can be time-consuming and error-prone for larger ones.
  2. Automated Tools: Use tools like PurifyCSS, UnCSS, or PurgeCSS. These tools analyze your website's HTML and JavaScript to determine which CSS selectors are actually used and then remove the unused ones from your CSS files.
  3. Modular CSS: Adopting a modular CSS architecture like BEM (Block Element Modifier) can help in maintaining smaller, more manageable stylesheets that are less likely to accumulate unused rules.


Before using PurgeCSS, your CSS file might look something like this:

.unused-class {
  color: blue;
.used-class {
  color: red;

After applying PurgeCSS, assuming .unused-class is not used anywhere on your site, the tool will remove it:

.used-class {
  color: red;

Implementing Preloading CSS

Preloading CSS is relatively straightforward and can be done by adding a rel="preload" attribute to your <link> tags in the HTML document's <head> section. This tells the browser that the linked resource is important for the current page and should be loaded as soon as possible.


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

In this example, critical-style.css is preloaded. The onload event changes the rel attribute to stylesheet after it's loaded, ensuring it's applied to the page. The noscript tag is a fallback for users with JavaScript disabled, ensuring they still receive the styled content.

Benefits of Reducing Unused CSS and Preloading CSS

  • Improved Load Times: By removing unnecessary bytes from CSS files, the browser can download and process stylesheets faster. Preloading critical CSS ensures that important styles are applied immediately, reducing render-blocking resources.
  • Enhanced User Experience: Faster load times lead to a smoother and more responsive user experience, crucial for keeping visitors engaged and reducing bounce rates.
  • SEO Advantages: Search engines favor websites that load quickly and provide a good user experience, potentially improving your site's ranking in search results.
  • Reduced Bandwidth Usage: Smaller CSS files use less bandwidth, which can be particularly beneficial for users on limited data plans or slow network connections.


Optimizing your website's performance is essential, and reducing unused CSS along with preloading critical CSS are effective strategies to achieve this goal. While the process may require some effort and maintenance, especially in identifying and removing unused CSS, the benefits in terms of improved load times, better user experience, and potential SEO advantages are undeniable. By implementing these techniques, developers can significantly enhance their website's efficiency and effectiveness, making it more appealing to both users and search engines alike.