What is Critical CSS?

What is Critical CSS?

Critical CSS refers to the minimum amount of CSS code that is necessary for the initial rendering of a webpage. This includes the CSS rules that are required to display the content above the fold, which is the area of the page that is visible to the user without scrolling. Critical CSS is essential for fast page loads because it allows the browser to render the content quickly, even before all the CSS files have been downloaded.

Why is Critical CSS important?

1. Faster page loads: By only loading the critical CSS, you can reduce the amount of data that needs to be downloaded, which can significantly speed up page loads.
2. Improved user experience: When users can see the content quickly, they are more likely to engage with your site, reducing bounce rates and improving user satisfaction.
3. Better SEO: Search engines also take page load times into account when ranking websites, so optimizing critical CSS can improve your search engine rankings.

How to optimize critical CSS?

Here are some steps to help you optimize critical CSS:

1. Identify critical styles: Identify the CSS styles that are necessary for the initial rendering of your webpage. This typically includes styles for:
   • Fonts
   • Colors
   • Text alignment
   • Backgrounds
   • Layout
2. Use a critical CSS builder tool: There are several tools available that can help you identify and extract critical CSS, such as:
   • Critical: A popular tool that generates critical CSS based on your HTML file.
   • PurifyCSS: A tool that minifies and optimizes your CSS code.
   • WebPageTest: A tool that provides a report on your website's performance, including critical CSS.
3. Use a preprocessor: If you're using a preprocessor like Sass or Less, you can write your critical CSS in a separate file and then concatenate it with your main CSS file using a tool like Gulp or Webpack.
4. Include critical CSS in your HTML: Include the critical CSS in your HTML file using the <style> tag or by linking it as an external stylesheet.
5. Use a CDNs or caching: Consider using a CDN or caching mechanism to serve your critical CSS files, as this can reduce the load time and improve performance.

Best practices for critical CSS

1. Keep it small: Keep your critical CSS code as small as possible to reduce the amount of data that needs to be downloaded.
2. Use relative units: Use relative units (e.g., rem, %, em) instead of absolute units (e.g., px) to make your styles more flexible and adaptable.
3. Avoid complex selectors: Avoid using complex selectors or complex layout rules in your critical CSS to keep it lightweight and efficient.
4. Use JavaScript wisely: Only use JavaScript to add dynamic styles or interactions once the page has loaded, rather than using it to apply styles at load time.

By following these best practices and optimizing your critical CSS, you can significantly improve the performance and user experience of your website.