Critical CSS (Cascading Style Sheets) is a technique used in web development and performance optimization to improve the loading speed and perceived performance of web pages. It involves extracting and delivering the essential CSS required to render the above-the-fold content of a web page, which is the portion of the page that is initially visible to users without scrolling.
When a web page is loaded, the browser needs to download and parse the CSS to apply the styles and render the content. CSS files can sometimes be large and complex, leading to a delay in page rendering, especially on slower internet connections or less powerful devices. This delay can result in a slower page load time and a suboptimal user experience.
Critical CSS addresses this issue by isolating the CSS rules that are necessary to style the above-the-fold content. Above-the-fold content typically includes the page header, navigation bar, hero section, and other elements visible without scrolling. By identifying and extracting the CSS rules specifically relevant to this above-the-fold content, the Critical CSS is generated.
The Critical CSS is then inlined directly into the HTML of the web page, either in the head section or as part of the inline CSS for the above-the-fold content. By doing so, the browser can immediately apply the critical styles without waiting for the external CSS file to download and parse. This results in faster rendering of the above-the-fold content, enhancing the perceived page load time and providing a better user experience.
The remaining non-critical CSS is usually loaded asynchronously or deferred to allow the browser to fetch and apply them after the initial page render. This allows the rest of the page to load more gradually and does not block the user from interacting with the content.
Implementing Critical CSS requires careful consideration of the specific CSS rules needed for the above-the-fold content, as well as regular updates to adapt to changes in the page layout and design. Several tools and plugins are available to automate the process of generating Critical CSS for websites.
Overall, Critical CSS is a valuable technique for optimizing web page performance, reducing initial load times, and delivering a smoother user experience by prioritizing the rendering of the most critical parts of the page.