Critical CSS

Christian Schreiber

Web Performance Consultant


What is Critical CSS?

Critical CSS refers to the minimum CSS rules that are necessary to correctly display the content of the visible area (viewport) of a website. This is the part of the CSS that is required so that the user can see and use the page as quickly as possible.

How does Critical CSS work?

The concept of Critical CSS is based on the idea of quickly loading and rendering only the CSS that is required for the visible part of a website. The main CSS code is divided into two parts: the critical CSS, which is embedded inline in the HTML code, and the remaining CSS, which is loaded asynchronously.

Why is Critical CSS important?

The integration of Critical CSS helps to optimize loading times by minimizing the so-called “render blocking” effect. This allows the browser to start rendering the page faster, resulting in an improved user experience.

How is Critical CSS created?

Various approaches and tools can be used to create Critical CSS. Here are some common methods:

Manual creation

The manual creation of Critical CSS requires in-depth knowledge of CSS and the structure of the website. This identifies and extracts the necessary CSS rules that are required for the visible area of the page.

Automatic tools

There are numerous tools that help to generate Critical CSS automatically. Examples include “Critical” by Addy Osmani, “Penthouse” and “”. These tools analyze the website and extract the necessary CSS rules automatically.

What are the advantages of using Critical CSS?

The implementation of Critical CSS has several advantages:

Faster loading times

By minimizing the render blocking resources, the browser can display the page faster, which significantly reduces loading times.

Better user experience

Users can see and use the page faster, which reduces the likelihood of bounces and increases dwell time.

Improved SEO

Google rates the loading time of a website as an important ranking factor. Faster loading pages therefore have a better chance of being ranked higher in the search results.

How can Critical CSS be optimized?

In order to exploit the full potential of Critical CSS, a few optimization methods should be observed:

Avoidance of overloading

Critical CSS should only contain the necessary styles to render the first view of the page. Overloading can negate the advantages.

Asynchronous reloading

The remaining CSS that is not critical should be loaded asynchronously so as not to block the main browser thread.

Regular review

As the content and layout of a website can change, the Critical CSS should be regularly reviewed and adjusted to ensure it remains effective.

What tools are available for creating Critical CSS?

There are various tools and libraries that support the creation and implementation of Critical CSS:

Critical by Addy Osmani

A Node.js tool that helps to extract and integrate Critical CSS.


Another tool for extracting critical CSS that can also be used to automate build processes.

An online solution that makes it possible to generate Critical CSS for any website by entering the URL.

How is Critical CSS integrated into WordPress?

Critical CSS can be integrated into WordPress with the help of plugins or manual adjustments:


There are several plugins that facilitate the implementation of Critical CSS, such as “Autoptimize” or “WP Rocket”.

Manual integration

For a customized solution, developers can manually integrate the Critical CSS into the WordPress theme by inserting it inline in the HTML header.


Critical CSS is an effective method for optimizing the loading times of websites. By identifying and integrating the minimum necessary CSS rules for the visible area of a website, the rendering time can be significantly reduced and the user experience improved. Various tools and methods are available to create and implement Critical CSS, especially in CMS systems such as WordPress. Regular reviews and optimizations of the Critical CSS are necessary to maintain the benefits in the long term.

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