WordPress critical CSS: How it affects your site?

Brian Bojan Dordevic
About The Author

Brian Decoded

President at Alpha Efficiency

Join me at the forefront of web design and digital marketing innovation. I am obsessed with web design, business philosophy and marketing performance.
I write Conversion Insider newsletter.

If you have used the Google PageSpeed Insights tool to audit a website, chances are that you have faced the “eliminate render-blocking resources” suggestion. As maximizing a website’s performance in search engines and with users is vital for every business, you should take these suggestions seriously. To solve this specific issue, you should aim to generate critical CSS for your WordPress website.

In this article, our web development agency in Chicago will cover what critical CSS is, how critical rendering path optimization can help you improve page speeds, and how you can do it on your website.

critical css in wordpress featured image
Table of Contents

What is Critical CSS?

By default, browsers don’t render page content before loading a complete CSS file. If your WordPress CSS file is large, your page speed can be really slow. As most users will bounce back to the SERPs if your website doesn’t load within 3 seconds, you can lose a lot of business opportunities unless you apply the proper techniques for instructing browsers to render your web pages and show content to visitors before they fully load CSS files.

Critical rendering path optimization is a technique used to improve page performance by extracting above-the-fold content, which makes pages render much faster for users. Critical CSS is responsible for rendering the content that visitors immediately see once they arrive on your website without scrolling. Keep in mind that WordPress critical CSS is not set up by default, which is why you need to invest efforts into optimizing CSS delivery to improve the user experience for your visitors.

When optimizing CSS delivery, it is essential to understand which elements are always critical, which elements are critical sometimes, and which elements are never critical.

Elements that are always critical include:

  • Background color
  • Theme dimensions
  • Above-the-fold page styling

While the above CSS is almost always critical, there are some CSS important only for specific users. For example, desktop layouts are not important to mobile users, as they are not necessary for properly rendering the mobile version of a website. That’s why elements that are critical only in specific circumstances include:

  • Menu styles
  • Footer design
  • Desktop layouts
  • Mobile layouts
  • Certain Page types

The following elements are not critical, as it is not vital to load them immediately in order to provide a good user experience:

  • Below-the-fold CSS
  • CSS that is meant for other types of devices than the one being used in the current browsing session

Understanding WordPress CSS

Before we go into details on how you can generate critical CSS, let’s try to understand WordPress CSS better. First, we should make the difference between external CSS and internal CSS:

External CSS

External CSS is a style sheet not embedded in the document. Instead, it is placed on a web server and referenced by the document’s HTML code. To use an external stylesheet, you need to link to it in the <head> section of your HTML code. That allows you to override the styles that are applied by the browser. External CSS can be used for various purposes, such as customizing the look and feel of a website for different devices or adding your own customizations. External CSS stylesheets are also easier to maintain and work with as it allows you to use the same stylesheet for multiple pages. However, keep in mind that your pages may render correctly only after the browser downloads the complete external CSS file. 

Internal CSS

Internal CSS is placed in the HTML file with a <style> tag. While using internal stylesheets can be challenging to maintain, it opens opportunities for critical CSS delivery. Internal CSS is also effective if you want to edit a single page of your website. 

Why Critical CSS Matters?

Before visualizing a page, the browser has to load, parse and execute all CSS files referenced in the HTML document. Larger CSS files will take longer to process, resulting in slower page loading times for all website visitors. Sometimes WordPress points browsers to multiple stylesheets, and the content will show to users after each of them is downloaded and executed. That will not only cause poor user experience across your web pages but may also lead to lower rankings for your website in search engine results pages due to the recent Core Web Vitals update. But what can you do to keep visitors and search engines satisfied?

You can place critical CSS in the <head> section of your HTML document instead of placing it in an external stylesheet and help browsers visualize the page for users faster. That will also make your external CSS file smaller in size, which will also result in better page speeds. Even though the entire page might take a long to load, users will almost immediately have content to interact with, so they will be less likely to bounce back to the search engine results page.

How to Generate Critical CSS in WordPress Site?

Considering the deep connection between site speed and SEO, utilizing critical rendering path optimization for improving a website’s performance is crucial for businesses that want to provide a great user experience and boost their rankings in search engine results pages. But how exactly can you generate critical CSS in a WordPress site? There are two approaches to this:

Manually generate WordPress critical CSS

Generating WordPress critical CSS manually can be a difficult task, as you will have to inspect every page element while considering its style and repeat the process for each page of your website. You will also need to generate critical CSS for different viewports in order to optimize the experience for both desktop and mobile visitors. We recommend avoiding this approach, as it is very time-consuming and error-prone. 

Perform content delivery optimization with the WP Rocket plugin

Manually generating WordPress critical CSS can be challenging, and that’s why we advise you to use a critical CSS plugin for this purpose. There are several critical CSS WordPress plugins that can help you pass Core Web Vitals more easily. However, when it comes to performing critical rendering path optimization, our Chicago SEO agency recommends using the WP Rocket plugin. This plugin can help you quickly generate WordPress critical CSS, which is necessary for rendering above-the-fold content, but also to prioritize in which order all the other CSS files should be loaded.

After you install WP Rocket, navigate to the ‘File optimization’ tab and then select the ‘Optimize CSS delivery’ box. The next step is to click on the ‘Remove Unused CSS’ option and let WP Rocket perform critical rendering path optimization and take care of the WordPress critical CSS for you. Now you can use the Google PageSpeed Insights tool and see how WP Rocket has helped you improve your score. 

Steps to Optimize CSS Delivery

To maximize the performance of your website and achieve above average PageSpeed score, besides generating WordPress critical CSS, you need to optimize CSS delivery. There are two main rules you should pay close attention to if you want to optimize CSS delivery on your website successfully. First, you should structure the HTML code to prioritize loading the above-the-fold content as fast as possible. Secondly, you should minimize the amount of data in your HTML, JS, and CSS files. Here is a step-by-step guide you can follow:

Test your website

Before you start optimizing CSS delivery, it is essential to check your website performance by using the Google PageSpeed Insights tool. If you see the eliminate render-blocking resources suggestion, approach the next step of this guide.

Combine CSS scripts into one script

As a large number of external CSS files can slow down your website loading times, we recommend combining all of your CSS scripts into one script that will be bigger in size. That will help you reduce the number of requests the user’s browser has to handle; therefore, it is a vital part of critical rendering path optimization and boosting WordPress CSS delivery.

Compress a new CSS script

The new CSS script you have created will probably be big in size, so it is crucial to compress it to reduce the amount of data users have to download when accessing your pages.

Adjust CSS rules to prioritize above-the-fold content

This is the crucial part of optimizing CSS delivery for your website. Extract the critical CSS responsible for above-the-fold content from your large CSS file, include it in the HTML <head> tag, and defer load the rest of the script. That will allow browsers to show content to users before executing all the WordPress CSS files.

Avoid including ‘style’ tags in the HTML body

The next step you need to take in order to eliminate render-blocking resources successfully is to remove style tags from the HTML body and place these lines of code in the <head> section of the HTML. You can manually check whether there is any style tag in the body of your HTML code or use a tool to speed up the process.

Don’t use the CSS @import rule

This rule allows you to import external CSS in your CSS script. However, this is bad practice for those who aim for high page speeds, as external files will be loading separately. By avoiding CSS @import rule, you will also reduce the number of unnecessary HTTP requests.

Website Speed and Critical CSS

People start forming opinions about businesses much before they get a chance to explore their content. The buying journey starts as soon as they click on your website on the search engine results page. How fast your page loads is one of the first factors, they consider when forming the first impression about your business. And as the first impression is usually the lasting impression, you should do everything in your power to improve your website’s speed and make your page layout load as fast as possible.

No matter how big or small your CSS file is in size, optimizing CSS delivery can help you boost page speed. This approach allows you to prioritize the rendering of critical CSS elements and instruct browsers to proceed with executing the rest of the CSS files only after the content is visualized for users.

Does critical CSS improve performance?

Generating critical CSS can help you improve your overall website speed, but you will see the most noticeable change in the First Contentful Paint metric. This metric is especially important, as it will help you improve the perceived speed of your website. Critical CSS allows browsers to visualize above-the-fold content for users much faster and then move on to loading the rest of the CSS script. That not only improves user experience and reduces bounce rates but also creates a specific “illusion” about your overall page speed, making you appear more professional to potential customers. 

However, keep in mind that WordPress critical CSS won’t improve your search engine rankings on its own. It is just a tiny piece of a much bigger puzzle you need to solve in order to outperform your competitors online. 

Get in touch

Are you having trouble maximizing the performance of your website? You should consider hiring a team of experts with years of experience building and maintaining successful business websites. Get in touch with our digital marketing agency today, and we will audit your website to find issues and apply the right solution.


Fresh inspiration is a fingertip away,
Download Our Portfolio.

Download Our Portfolio