Critical CSS

Critical CSS is a web performance optimization technique that involves extracting and inlining the minimal CSS required to render the above-the-fold content of a webpage. This reduces render-blocking resources by delivering essential styles immediately in the HTML, while deferring non-critical CSS. It improves perceived page load speed and user experience by ensuring visible content appears quickly.

Also known as: Above-the-fold CSS, Critical Path CSS, Critical Rendering Path CSS, Inline Critical CSS, FCP CSS

Why learn Critical CSS?

Developers should use Critical CSS for performance-critical websites, especially content-heavy pages, e-commerce sites, and mobile applications where fast initial rendering is crucial. It's particularly effective in reducing First Contentful Paint (FCP) and Largest Contentful Paint (LCP) metrics, which are key factors in SEO rankings and user retention. This technique is essential for optimizing Core Web Vitals and improving overall site responsiveness.

Learning Resources

• docs: Google Developers: Critical CSS
• tutorial: CSS-Tricks: Critical CSS
• docs: Web.dev: Optimize CSS Delivery
• video: YouTube: Critical CSS Explained
• tutorial: Smashing Magazine: Critical CSS Tools