Understanding the Critical Render Path and Its Role in Web Performance

About the Author

This blog was written by Sagnik Ghosh. He is currently working at P360 as a Software Engineer. He has over 2+ years of experience on working on web development and app development. We’re grateful to have him share his insights and knowledge about Web Performance with our community. If you want to know more about Sagnik, check out his LinkedIn profile.

Introduction

Web performance is a critical factor in modern web development, directly impacting user experience, engagement, and conversion rates. The Critical Render Path (CRP) is a core concept in optimizing web performance. By understanding and optimizing the CRP, developers can significantly improve the speed at which web pages load and become interactive. Let’s dive into what the CRP entails, why it matters, and how to optimize it based on insights from web.dev.

Why Web Performance Matters

Speed is at the heart of web performance. According to web.dev, fast-loading websites lead to better user engagement, retention, and overall satisfaction. Slow websites, on the other hand, frustrate users, leading to higher bounce rates and lost opportunities. Performance isn’t just about milliseconds, it’s about creating seamless experiences that respect users’ time.

Google have spent some considerable amount of time creating user centric performance metrics which are known as Core Web Vitals. Here is a list of some case studies which highlights the connection between performance and business outcomes based on these core web vitals -

The Role of the Critical Render Path

The Critical Render Path refers to the sequence of steps the browser takes to convert HTML, CSS, and JavaScript into pixels on the screen. Every web page has its unique CRP, influenced by the content structure, resource size, and dependencies. Optimizing the CRP ensures a faster path from the server to the user’s viewable content.

Key Stages in the Critical Render Path

HTML Parsing: The browser parses the HTML document to construct the Document Object Model (DOM).
CSSOM Construction: The browser processes CSS files to build the CSS Object Model (CSSOM).
JavaScript Execution: If the HTML references external or inline JavaScript, the browser may pause parsing to execute scripts.
Render Tree Construction: The DOM and CSSOM combine to form the Render Tree, representing elements to be displayed with their styles.
Layout and Paint: The browser calculates the layout of elements and paints pixels on the screen.

(Content of this Image is sourced from web.dev)

Only after all these steps have been completed, will the user see content on the screen. The critical rendering path focuses on the process outlined for the initial render, and depends on the critical resources necessary for it. And in between re-rendering happens multiple times as more resources which affect page's rendering becomes available to update what the user sees.

Resources on the Critical rendering path

Here’s an overview of the resources that influence the critical rendering path and how browsers handle them -

1. Critical Resources for Initial Rendering

The browser requires certain resources to complete the initial page render, including:

HTML: Key portions are processed as they arrive.
Render-blocking CSS: Typically in the <head> element.
Render-blocking JavaScript: Typically in the <head> element.

2. What the Browser Doesn’t Wait For

For efficiency, the browser doesn’t halt the initial render for:

All HTML: Rendering starts with the available portions.
Fonts: Text may be temporarily invisible until fonts load.
Images: Space may be reserved, but the browser moves forward.
Non-render-blocking JavaScript: Often placed outside the <head> or marked as async or defer.
Non-render-blocking CSS: Includes styles with media attributes irrelevant to the current viewport.

3. The Role of the `<head>` Element

The <head> contains metadata and critical resources that guide rendering. It does not include visible content but ensures the browser knows how to render the visible content in the <body>element.

4. Render-Blocking Resources

Render-blocking resources pause rendering until they’re downloaded and processed:

CSS: Blocks rendering by default unless its media attribute makes it non-applicable to the current conditions.
Recent Innovations: The blocking=render attribute allows developers to explicitly manage rendering-blocking elements.

What is FOUC?

CSS is considered a render-blocking resource because it prevents the browser from displaying any content until the CSS Object Model (CSSOM) is fully constructed. This behavior ensures a seamless user experience by avoiding a Flash of Unstyled Content (FOUC), where the page appears briefly without styling before the CSS is applied.

While FOUC is typically brief and may go unnoticed, understanding its implications is essential. The browser delays rendering to maintain a polished appearance, but prolonged render blocking can negatively impact performance. To address this, ensure your CSS is efficient and well-optimized to reduce the time it takes for the page to display fully styled content.

Here is a sample video clip of FOUC taken from web.dev where you can see the page without any styling and all styles are applied once the page's CSS has finished loading from the network.
Example of FOUC

5. Parser-Blocking Resources

These resources halt the browser’s ability to parse HTML further:

JavaScript: By default, it blocks parsing unless marked async or defer. This ensures DOM and CSSOM stability before execution.

Browsers mitigate parser blocking with techniques like the preload scanner, which downloads resources in advance to reduce delays.

6. Identifying Blocking Resources

Tools like WebPageTest and Lighthouse help developers pinpoint performance bottlenecks:

WebPageTest: Marks render-blocking resources with an orange circle in its network waterfall diagram.
Lighthouse: Highlights resources only if they delay rendering, avoiding false positives.

Optimizing the Critical Render Path

So now that we have understood what a Critical Rendering Path (CRP) is and how different resources affect it's path while rendering, It's time to know how to optimize the CRP for better web performance, ensuring faster page load times, and enhancing user experience. Below are key strategies to streamline the CRP -

1. Efficient HTML Structure

As highlighted in General HTML Performance, clean and semantic HTML is essential. Avoid excessive DOM depth and unnecessary elements, as they can complicate the rendering process. Use modern HTML standards to ensure compatibility and efficiency.

2. Minimize Critical Resources

Critical resources are those required to render the first paint of the page. To optimize their impact:

Eliminate unnecessary resources: Remove unused CSS, JavaScript, and large media files.
Inline critical CSS: Embed only the CSS needed for above-the-fold content directly into the HTML.

3. Optimize Resource Loading

Efficient resource loading can significantly reduce delays:

Use async and defer for JavaScript: These attributes prevent JavaScript from blocking the parsing of HTML.

async: Scripts load independently and execute as soon as they are ready.

defer: Scripts load in order and execute after the HTML parsing is complete.

Lazy-load non-critical resources: Images, videos, and other resources not immediately visible can be loaded only when needed.

4. Prioritize Critical Resources

Ensure the browser prioritizes the most important resources:

Preload key resources: Use the <link rel="preload"> tag for fonts, images, and other assets required early in the rendering process.
Reduce the size of critical resources: Minify CSS, JavaScript, and compress images to decrease file sizes.

5. Reduce the Number of Critical Path Length

Shortening the CRP involves reducing the number of render-blocking requests:

Combine CSS and JavaScript files: Fewer requests translate to shorter load times.
Eliminate render-blocking CSS and JavaScript: Defer or asynchronously load these files to allow the page to render sooner.

6. Optimize Web Fonts

Fonts can delay the first paint and add to the CRP:

Use font-display: swap: This CSS property ensures text is displayed using fallback fonts until the web font is fully loaded.
Preload fonts: Preload key font files to make them available earlier in the rendering process.

Measuring and Monitoring Performance

Optimization doesn’t end with implementation. Regularly measure performance using tools like Lighthouse or WebPageTest to analyze the CRP. Look for metrics such as:

First Contentful Paint (FCP)
Time to Interactive (TTI)
Largest Contentful Paint (LCP)

Identify bottlenecks and iteratively improve based on data.

Conclusion

The Critical Render Path is a vital aspect of web performance. By understanding its components and employing optimization techniques, developers can create faster, more engaging websites. Remember, performance is a journey, not a destination. Continuously monitor, test, and refine your strategies to stay ahead in delivering exceptional user experiences. If you like this blog and want to learn more about software engineering and related topics then follow GDG Siliguri on hashnode.