What Is Critical Rendering Path And How To Optimize It?

The critical rendering path refers to the steps of converting the fundamental resource files such as CSS, JavaScript, and HTML into a webpage. To improve your website performance, you need to optimize the critical rendering path by minifying resources, reducing unused CSS, Deferring JavaScript files, and using other optimization techniques.

Critical Rendering Path

As a website owner or developer, your first priority should be providing a well-functioning website. To do so, you need to understand and optimize the critical rendering path. We will discuss the critical rendering path and explore optimization techniques.

What Is a Critical Rendering Path?

Critical rendering path (CRP) indicates the sequence of the process the web browser takes to convert your HTML files, CSS stylesheets, and scripts into a webpage that your user can see and interact with. 

Before diving into the optimization process first you need to understand each process of the critical rendering path in sequence which includes:

  • Document Object Model (DOM).
  • CSS Object Model (CSSOM).
  • The Render Tree.
  • Layout.
  • Paint.

What is the Document Object Model (DOM)?

The Document Object Model (DOM) is an application programming interface for web documents. It allows you to add, modify, or delete elements from your web document. This DOM views your HTML elements as nodes. 

Document Object Model, Critical rendering path

Let’s understand with an HTML code.

<!DOCTYPE html>

<html lang="en">


    <meta charset="UTF-8">

    <meta name="viewport" content="width=device-width, initial-scale=1.0">

    <meta http-equiv="X-UA-Compatible" content="ie=edge">

    <title>DOM tree structure</title>



    <h1>DOM tree structure</h1>

    <p>Learn about the DOM constuction</p>


  • Here, <html> is presented in the root note and contains two children: <head> and <body>
  • <head> node has two children: <meta> and <title>
  • <body> also consists two children: <H1> and <H2>

Notes: When you are adding any elements to your website, this DOM size will be increased automatically. 

What is the CSS Object Model (CSSOM)?

CSS Object Model (CSSOM) is a set of  Application Programming Interface (API) that allows CSS of the website to be manipulated from JavaScript files.

cssom, critical rendering path

In simple words, it allows you to change the style rules of a document from a scripting file like JavaScript, enabling dynamic styling on your web page. Let’s understand with an example.

function css_object_model() {

 var button = document.getElementById(‘button’');

 button.style.color =”blue”;


In the above JavaScript file, we are changing the button color. When someone clicks the button, it will be blue. Therefore, this CSS object model makes dynamic staling possible on the web page. 

What is the Render Tree?

The render tree is a combination of the DOM and CSS Object Model (CSSOM). If any style content is defined for a given node of DOM and CSSOM, then style properties are assigned to each node in the render tree.

render tree, critical rendering path

It’s important to note that only the visible content is added to the render tree during its construction process. The head segment will not be present in the render tree because it is not visible.

For example:

<!DOCTYPE html>

<html lang="en">


    <meta charset="UTF-8">

    <meta http-equiv="X-UA-Compatible" content="ie=edge">

    <title>Render Tree structure</title>



    <h1>DOM tree structure</h1>

    <h2>Learn about the Render Tree structure</h2>



For the above HTML document, only <h1> and <h2> tags are added in the render tree. 

What is the Layout?

Once the render tree is constructed properly, the layout comes into the scene. It depends on the screen size. The layout determines the arrangement and size (width and height) of the elements in the web page. 

The DOM tree’s nodes determine how the layout is executed. The more node numbers, the longer it takes the browser to generate the layout. The algorithm that computes and makes use of a number of variables, positioning, floats, and the box model is known as the Visual Formatting Model.

What is the Paint?

Paint is the final step of this critical rendering path. During the page loading process, the pixels on the screen are painted. This paint time depends on the render tree structure.

You need to remember that, the render tree’s size will determine how long the paint stage takes.

How To Optimize Critical Rendering Path?

Optimizing the critical rendering path performance can improve your page speed, leading to a good conversion ratio and Search Engine Optimization (SEO) rankings. Let’s explore the optimization techniques for the critical rendering path.

Generating critical CSS

Critical CSS extracts the minimal set of CSS rules for rendering above-the-fold content faster. By generating critical CSS, you can improve the DOM size by reducing the nodes and optimizing the critical path rendering performance. For more information, go to our previous blog.

Remove unused CSS

Unused CSS styles or rules are present in your CSS file, but you are not currently using these on your website. These files increase the size of your CSS file and the DOM tree.

remove unused css, critical rendering path

Therefore, removing unused CSS can optimize your critical render-blocking path by deleting the unnecessary stylesheets and rules from the CSS file.

Deferring JavaScript

JavaScript files block your main thread, increasing the render-blocking resources Total Blocking Time (TBT). Deferring Javascript files is one of the optimization techniques that can improve your loading time by delaying the execution of JavaScript files.

To do so, you just need to add a defer attribute in the element that you want to defer. Here is the syntax of deferring the script tag.



<script src = "myscript.js" defer>  





<h1> rabbitLoader.com </h1>  

<h3> This is an example of a defer attribute. </h3>  




The above three techniques are crucial for optimizing your critical rendering path. After trying these three techniques, if you want to further optimize your resource files, then minification and compression are the best options.

HTML, CSS, and JS file Minification

Minification is the optimization technique of removing unnecessary characters from your HTML, CSS, and JavaScript files to reduce their size. These unnecessary characters can be line breaks, spaces, or any large variable name.

For more details, read our blogs: Minify Resources

Compressing the Website’s Resources

Compression is the optimization process that replaces the repeated strings in a pointer to reduce the overall page size. Therefore, by compressing your website’s resources, you can easily optimize your web performance.

Optimizing Your Website’s Critical Rendering Path With RabbitLoader

If you are technically sound enough, you can try to implement the above strategies manually. Otherwise, the best practice is to use optimization tools like RabbitLoader. Let’s see how RabbitLoader optimizes your website’s critical rendering path.

CSS Optimization, critical rendering path
  • RabbitLoader identifies your critical resource, creates the critical CSS file, and reduces the file size by 98%. 
  • When you optimize your website’s page speed with RabbitLoader, you don’t need other minification tools for your HTML, CSS, and JavaScript files.
  • RabbitLoader will optimize your critical rendering path by removing the unused CSS from your CSS files.


Here, we’ve discussed the critical rendering path and the optimization techniques to improve its performance. 

If you are thinking of improving your critical render path performance to boost your web performance, don’t waste your time. Install RabbitLoader and explore its features. The main advantage is the availability, especially if it’s a free version. 


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3 Best Plugins to Remove Unused CSS from Your WordPress Website and Improve PageSpeed.

Reduce unused CSS (previously remove unused CSS) is one of the most challenging suggestions triggered by Google pagespeed insights when you are auditing your website’s pagespeed performance

What is unused CSS? And how to remove this?

Unused CSS code is any CSS style or rules that you don’t need. Removing this unused CSS rule can improve your website’s page speed performance for a better conversion rate as well as better SEO rankings. 

how to remove unused css

Don’t worry! If you are not from a technical background. Here we will discuss the strategies that will help non-coders like you and thousands of other wordpress website owners to remove unused CSS.  

What is Unused CSS

Unused CSS refers to the stylesheets or style rules that are present in your website’s codebase but not applied to any elements on the webpage.

remove unused css

Unused CSS can occur due to several reasons such as design changes, code updates, or unused stylesheets from the CSS framework or library. This unused CSS makes the file larger than necessary and increases the loading time of your website. 

Type of Unused CSS:

Unused CSS generally falls into two categories. Both categories will affect your pagespeed performance in different ways.

  • Non-Critical CSS:

Non-critical CSS is also known as deferred CSS. It refers to a stylesheet that is not necessary for rendering the visible area or above the fold.

non critical css
  • Dead CSS :

Dead CSS refers to the CSS stylesheets that are no longer applied or used to elements on a website. They are basically remnants from the previous version of your website or maybe a part of testing or temporary usage.

Why Should You Remove Unused CSS File

Unused CSS stylesheets that are not needed to render a page, can be as files or as internal styles for the page contents on a web page. These unused CSS files can have several performance impacts on your website, affecting page loading time as well as user experience. 

  • Unused CSS Increases Page Loading Time:

An unused CSS makes a file larger, which will increase the web page loading time for a user. This is especially critical for the user who is using a slower network connection.

Increase Page Loading Times

Larger CSS files increase the network overhead browser needs to download more data. This can be problematic for your website to attract high traffic.  

  • Unused CSS Impacts on LCP Negatively:

Non-critical CSS also affects the core web vitals, especially on Largest Contentful Paint (LCP), making it lengthier which drags your pagespeed score down.

csimpacts on lcp

LCP is a user experience (UX) metric that indicates the time length taken to load a large content such as images, text blocks, etc. To provide a better user experience it should be less than 2.5 seconds.

  • Delayed Browser Rendering:

When a user visits any website, the browser will download, parse, and process the content of the website before displaying or rendering the webpage content. Hence, whether used or not, the browser will download each CSS file from the server.

Browser rendering process

A delayed render ,means user experience would be far from optimal.

  • Mobile Performance Issue:

Mobile users often have limited resources compared to desktop users. The desktop has a better internet speed than the mobile. It has a better processor and it can be connected to the Internet with the help of cables.

mobile performance issue, remove unused css

Whereas the mobile uses the data network and wireless connections. So, they may experience more issues when downloading and rendering large CSS files.

Check out: Core Web Vitals

How to Remove Unused CSS Manually

Removing the unused CSS file manually is a meticulous process and a piece of coding knowledge is required for this. Here we are mentioning some strategies that you can follow.

  1. Audit Your Code to Remove Unused CSS:

You need to go through the whole HTML file and identify which CSS classes and IDs are not being used. It can be done by searching for a specific class or ID in your HTML code.

audit unused css

This process is very time-consuming. You can also use any browser developer tool such as Chrome DevTool or Firefox Developer Edition. These tools allow you to identify unused CSS rules. The CSS that is solid red indicates the code is unused.

  1. Remove Unused CSS Styles:

Once you have identified the unused CSS codes, remove these codes from your style sheet. Be very careful, not to delete the CSS rule that may be used conditionally.

Style code unused
  1. Test Your Changes:

After removing all unused CSS rules from your stylesheets, you need to test your website once to ensure that no visual or functional issue has arisen.

Tools to Remove Unused CSS 

There are several online tools available to remove unused CSS files to improve web performance. These tools can help you to analyze your CSS stylesheets and extract only the used CSS rules. Here is an overview of the popular tools.

1. PurifyCSS:

PurifyCSS is a free online tool that helps to remove unused CSS from your HTML, JS file, and CSS files. But you need to manually specify which files need to be scanned one by one.

Purify css online,remove unused css

PurifyCSS provides new files with optimized CSS rather than modifying the original files. Since it doesn’t work with any live website, you need to use it in development settings.  

2. UnusedCSS:

UnusedCSS is a user-friendly online tool for removing unused CSS. you just need to plug the website’s URL into UnusedCSS. It will identify and remove unused CSS code and also tell you how much memory you have saved.

unused css

You can try this tool for free but you need to pay for a monthly membership to download the optimized files. 

3. PurgeCSS:

PurgeCSS looks very similar to PurifyCSS which helps you to analyze your webpage content and your CSS files. It removes the unused CSS selector from your CSS and provides a smaller file.  

Purge css

Best Plugins to Remove Unused CSS for WordPress Websites.

The easiest way to remove unused CSS is to use one of the WordPress optimization plugins that can help you remove unused CSS. There are many optimization plugins available that can help you. Here we are mentioning a few popular WordPress plugins.

1. RabbitLoader:

RabbitLoader is an all-in-one performance optimization plugin that you can use to remove unused CSS features. Apart from this RabbitLoader uses various techniques like code optimization, image optimization, a premium CDN, and many more to speed up your website. 

rabbitloader,remove unused css

When you are using RabbitLoader no other plugin is required. The advantage of RabbitLoader over the other optimization tools is the availability of a free version.

2. WP Rocket:

WP Rocket is the most popular WordPress plugin that optimizes your website. It uses various optimization techniques like minification of CSS files and unused JavaScript files, caching, and others. 

WP rocket

But when you are using WP Rocket in your WordPress site you need to install Imagify for image optimization, a premium CDN that is known as RocketCDN, and for lazy loading, you need Rocket lazy loading. It is only available in the WordPress ecosystem.

3. Asset Cleanup:

Asset cleanup is a plugin that helps you to reduce unused CSS to improve the WordPress website’s pagespeed. First, it will fetch the homepage and will show you all CSS files loaded on that page.

asets cleanup, remove unused css

You need to review all loaded files. If you find any files that you don’t need then you need to unload that file.


We hope this article will help you to understand Unused CSS and guide you to remove unused CSS. If you are comfortable with coding then you can remove unused CSS manually. But if you are not a coder like me then you can install any optimization tools that can help you to reduce unused CSS.

If you are a beginner, We suggest you install the all-in-one performance optimization tool RabbitLoader which can help you to optimize your website pagespeed. It will improve the user experience as well as the conversion rate.