How Web Browsers Load Pages – The Critical Rendering Path Explained
How Web Browsers Load Pages – The Critical Rendering Path Explained
Published on March 25, 2025 Last Updated on April 30, 2025
Written by
Morgan Frank - Specialist in Page Speed
When you type a website address into your browser and press Enter, a series of steps happen behind the scenes to display the webpage. This process is called the Critical Rendering Path.
It’s a sequence of actions the browser must take to turn the website’s code into the visual page you see. Understanding this process is key to making websites load faster.
Key Takeaways From The Survey
Here’s what we’ll explore:
The Critical Rendering Path is the sequence of steps a browser takes to turn HTML, CSS, and JavaScript into a visible webpage.
The main steps are: Request, Response, building the DOM, building the CSSOM, creating the Render Tree, Layout, and Paint.
HTML, CSS, and JavaScript all play a role, and each can potentially slow down the process.
Understanding this path is the foundation for making your website faster.
The Steps to Display a Webpage
Let’s break down the Critical Rendering Path step-by-step, without any complicated jargon. You can understand how web browsers are load pages.
Step 1: The Request
You enter a website address (URL) into your browser and press Enter.
The browser sends a request to the server that hosts the website. This is like asking the server for the website’s files.
DNS Lookup: Before the browser can request the page, it needs to find the server’s exact address (its IP address). It uses a system called DNS (Domain Name System) to translate the website name (like “example.com”) into an IP address (like “192.168.1.1”). It is similar to look up their number in the phone directory.
Step 2: The Server Responds
The server receives the request and sends back the website’s files. The most important files are:
HTML (HyperText Markup Language): This file contains the basic structure and content of the webpage – the text, headings, image information, and links.
CSS (Cascading Style Sheets): This file contains the styling rules for the webpage – how the text should look (font, color, size), how elements should be arranged, and the overall visual design.
JavaScript (JS): This file adds interactivity to the webpage – things like animations, handling button clicks, and updating content dynamically.
Step 3: Building the DOM (Document Object Model)
The browser starts by reading the HTML file.
It converts the HTML code into a structure called the DOM (Document Object Model). The DOM is like a tree where each part of the HTML (like a heading, a paragraph, or an image tag) becomes a “node” in the tree. This tree represents the structure of the webpage.
Important: The browser needs to finish building the DOM before it can display anything on the screen.
Step 4: Building the CSSOM (CSS Object Model)
As the browser reads the HTML, it also finds the CSS file (or CSS code within the HTML).
It converts the CSS code into another structure called the CSSOM (CSS Object Model). The CSSOM is also like a tree, but this tree describes the styles that should be applied to the different parts of the webpage. For example, it might say, “Make all headings blue and large.”
Important: The browser needs the complete CSSOM to know how to style the page correctly. CSS is “render-blocking,” meaning the browser won’t show anything until it has processed the CSS.
Step 5: Creating the Render Tree
The browser combines the DOM and the CSSOM to create something called the Render Tree.
The Render Tree only includes the parts of the webpage that will actually be visible on the screen. If an element is hidden using CSS (like display: none;), it won’t be in the Render Tree.
Key Point: The Render Tree is essential. The browser cannot start drawing the page until this tree is built.
Step 6: Layout
Once the Render Tree is ready, the browser performs the Layout step (sometimes called “Reflow”).
During Layout, the browser calculates the exact size and position of each element on the screen. It figures out where everything should go, based on the styles in the CSSOM and the structure of the Render Tree.
Layout Can Be Slow: This step can take a lot of processing power, especially for complex webpages.
Step 7: Paint
Finally, the browser paints the pixels onto the screen. This is the step where the actual visual content appears. The browser uses the information from the Render Tree and the Layout calculations to draw everything.
Rasterization: Converts all vectors to pixel format.
Compositing: Overlapping elements are managed by the browser.
Step 8: JavaScript Execution
JavaScript code can change both the content (DOM) and the styles (CSSOM) of the webpage. This is how websites become interactive.
JavaScript is often “parser-blocking.” When the browser finds a <script> tag in the HTML, it usually stops processing the HTML and waits for the JavaScript to download and run. This is because the JavaScript might change the DOM, and the browser needs to know about those changes. (There are ways to optimize this, which we’ll discuss later.)
Execution: Browser will execute the JavaScript code.
Why is this “Critical”?
This process is called the Critical Rendering Path because these steps are absolutely necessary to display the webpage. If any step is slow, the user will see a delay. Optimizing the Critical Rendering Path means making each of these steps as fast as possible.
Conclusion
The Critical Rendering Path might seem complicated at first, but breaking it down into these steps makes it much clearer. By understanding how the browser works, you can identify bottlenecks and make changes to your website’s code to improve its loading speed. This leads to a better experience for your users, better search engine rankings, and ultimately, a more successful website. We’ll cover specific optimization techniques in the following sections.
Determined to change that, he built RapidLoad — a smart, AI-driven tool that empowers site owners to dramatically improve speed scores, enhance user experience, and meet Google’s Core Web Vitals without needing to touch a single line of code.
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