Critical Rendering Path in my web development experience

by Alex Gurkin | Updated: Oct 18, 2023

As a web developer, understanding the critical rendering path is essential for creating high-performing websites. The critical rendering path refers to the sequence of steps that a browser takes to render a web page. This process involves fetching and loading resources, parsing HTML and CSS, and rendering the content on the screen.

1. Resource Loading

The first step of the critical rendering path is resource loading. This involves fetching the necessary resources such as HTML, CSS, JavaScript, images, and fonts. It’s important to optimize the loading of these resources by minimizing their size and reducing the number of requests the browser needs to make. This can be achieved through techniques like minification, compression, and caching.

2. HTML Parsing and DOM Construction

Once the browser has fetched the HTML file, it starts parsing the markup to understand its structure. During this process, the browser builds the Document Object Model (DOM) which represents the structure of the webpage. It’s crucial to write well-structured HTML code to ensure efficient parsing and DOM construction. Avoiding unnecessary nested elements and reducing the use of inline JavaScript and CSS can help improve performance.

3. CSS Parsing and Render Tree Construction

After parsing the HTML, the browser starts fetching and parsing the CSS files. The parsed CSS is then used to construct the render tree, which represents the visual elements of the webpage. The render tree contains only the elements that are necessary for rendering the page. It’s important to write optimized CSS code and avoid unnecessary selectors or complex styles that can slow down the rendering process.

4. Layout and Paint

Once the render tree is constructed, the browser goes through the process of layout and paint. During layout, the browser calculates the position and size of each element in the render tree. This is followed by the paint phase, where the browser fills in pixels with the appropriate colors based on the render tree and layout calculations. To optimize this process, it’s important to minimize layout thrashing and avoid excessive DOM manipulations.

5. Compositing

The final step in the critical rendering path is compositing. This is where the browser combines all the painted elements into layers and arranges them in the correct order. Compositing involves the GPU (Graphics Processing Unit) and can be accelerated by utilizing hardware acceleration and optimizing CSS properties like transforms and opacity.

Understanding the critical rendering path is crucial for web developers to create fast and efficient websites. By optimizing the loading of resources, writing well-structured HTML and CSS, minimizing layout thrashing, and utilizing hardware acceleration, developers can improve the rendering performance of their web pages and provide a better user experience.

A seasoned project manager & CEO since 2008. MD with medical IT skills. 300+ web projects. Published author. SaaS expert. read more

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