What does Critical CSS mean and how is it implemented?

Critical CSS refers to the minimum CSS code needed to correctly render the visible area of a page (above the fold) during the first load. This CSS code is embedded inline in the HTML document so the browser can immediately render the page without waiting for external CSS files. The remaining CSS is loaded asynchronously. Implementation involves automated extraction of critical CSS using specialized tools that analyze the visible area for different screen sizes. The result is a significantly faster First Contentful Paint.