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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