Using Inline CSS for Faster First Render

Cascading Style Sheets (CSS) tell web browsers how a web page is supposed to be displayed or rendered . In the early web, this information was written directly (i.e. inline) in the HTML that made up a web page. CSS information is now often stored in separate files that web browsers download when loading a web page. Storing CSS styling in separate files makes it possible for CSS information to be cached and to be loaded later in the web page loading process. However, downloading CSS files still takes additional time, especially for first-time web page visitors who do not have cached copies of the CSS files yet. If these CSS files contain the styling and information the web browser needs before it can start showing the web page, the visitor has to wait for that information to be downloaded before any web page content will render.

Using an HTTP/2 connection can improve web page loading performance; however, visitors with slow Internet connections or those located  farther away from where your website is hosted may still have to wait longer for the web page to render any content in their web browsers. For these users, putting the most important CSS styling inline can make web browsers start showing web page content sooner than it would if they had to wait for separate CSS files to download.

WordPress websites can use plugins to automatically add inline critical CSS styling rules. Knowledgeable WordPress administrators or WordPress developers can also manually add CSS styling inline into web pages using the text editor directly.