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Critical Rendering Path


The Critical Rendering Path (CRP) refers to the sequence of steps the browser takes to convert HTML, CSS, and JavaScript into pixels on the screen. It is a crucial aspect of web performance optimisation because it directly impacts how quickly users can see and interact with the content. The CRP involves the following steps:

  1. Document Object Model (DOM) Construction: The browser parses the HTML to construct the DOM tree.
  2. CSS Object Model (CSSOM) Construction: The browser parses the CSS to create the CSSOM tree.
  3. Render Tree Construction: The browser combines the DOM and CSSOM trees to form the render tree, which contains only the nodes required to render the page.
  4. Layout: The browser calculates the layout of each node in the render tree, determining its exact position and size on the screen.
  5. Painting: The browser paints the pixels to the screen based on the layout and styles.

Optimizing the CRP is essential for improving page load times and the overall user experience. By reducing the time it takes for the browser to construct and render the content, web developers can ensure that users see and interact with the page as quickly as possible.

How You Can Use Critical Rendering Path


Consider an online news website that aims to enhance its page load performance. By analyzing the Critical Rendering Path, the developers identify that several large CSS files and JavaScript scripts are blocking the rendering of above-the-fold content.

To optimize the CRP, the developers take the following steps:

  1. Minimise Render-Blocking Resources: They reduce the number of CSS and JavaScript files that need to be loaded before rendering the critical content. This is achieved by inclining critical CSS directly in the HTML and deferring non-critical JavaScript.
  2. Optimize CSS Delivery: By splitting the CSS into critical and non-critical parts, the critical CSS is loaded first to render above-the-fold content quickly, while the rest is loaded asynchronously.
  3. Lazy Load Images and Scripts: Non-essential images and scripts are loaded lazily to prevent them from blocking the initial rendering process.

Formulas and Calculations

To calculate the Critical Rendering Path length,

  1. Identify Critical Resources: Determine which resources are required for rendering above-the-fold content.
  2. Measure Resource Load Time: Calculate the time it takes to load each critical resource.
  3. Calculate Total Time: Sum the load times of all critical resources to determine the total Critical Rendering Path length.

For example, if the critical CSS takes 200ms to load, the critical JavaScript takes 150ms, and the HTML parsing takes 100ms, the total CRP length is 450ms.

Key Takeaways

  1. Optimisation: Reducing the number of render-blocking resources can significantly speed up the initial load time.
  2. Inlining Critical CSS: Including critical CSS directly in the HTML can reduce the time to first render.
  3. Defer Non-Critical JavaScript: Deferring JavaScript that is not essential for initial rendering can improve page load performance.
  4. Lazy Loading: Implementing lazy loading for images and scripts can help reduce the Critical Rendering Path length.
  5. Performance Monitoring: Regularly monitoring and analyzing the Critical Rendering Path can help maintain optimal web performance.


What is the Critical Rendering Path (CRP)?

The CRP is the sequence of steps the browser takes to convert HTML, CSS, and JavaScript into pixels on the screen.

Why is the Critical Rendering Path important?

Optimizing the CRP is crucial for improving page load times and user experience.

How can I optimize the Critical Rendering Path?

You can optimize the CRP by minimizing render-blocking resources, inlining critical CSS, deferring non-critical JavaScript, and implementing lazy loading.

What are render-blocking resources?

Render-blocking resources are files like CSS and JavaScript that must be loaded and processed before the browser can render the page.

What is inlining critical CSS?

Inlining critical CSS involves embedding the CSS needed for the above-the-fold content directly in the HTML to speed up the initial rendering.

What does deferring non-critical JavaScript mean?

Deferring non-critical JavaScript means delaying the loading and execution of JavaScript files that are not necessary for the initial rendering of the page.

How does lazy loading work?

Lazy loading delays the loading of non-essential resources, such as images and scripts, until they are needed, reducing the initial load time.

What tools can I use to analyze the Critical Rendering Path?

Tools like Google Page Speed Insights, Lighthouse, and Chrome Dev Tools can help analyze and optimize the CRP.

What is the render tree?

The render tree is a combination of the DOM and CSSOM trees, containing only the nodes required to render the visible content of the page.

How does the CRP affect SEO?

A shorter CRP can improve page load times, leading to better user experience and potentially higher search engine rankings.

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