Inlining critical CSS is a performance optimization technique that involves embedding the CSS necessary for rendering above-the-fold content directly into the HTML document. This eliminates the need for the browser to fetch external stylesheets before displaying critical content, leading to faster rendering and improved user experience. To achieve this, identify the CSS rules required for the initial visible portion of the webpage. These rules can be extracted using tools like Chrome DevTools, Critical, or PurifyCSS. Once identified, these styles are placed within a style block in the head section of the HTML, ensuring that they load immediately when the browser processes the document.

Render-Blocking Resources

After inlining critical CSS, the rest of the styles can still be loaded as an external stylesheet. However, these non-critical styles should be deferred or loaded asynchronously to avoid blocking the rendering process. By combining inlined critical CSS with deferred loading of non-critical styles, webpages can significantly reduce render-blocking resources, improving the time-to-first-paint (TTFP) and overall page load speed. This optimization is particularly beneficial for mobile users and those with slower network connections, as it enhances perceived performance and boosts metrics such as the Largest Contentful Paint (LCP) in Core Web Vitals.