Ultimate Guide to Fetch Priority API

The Fetch Priority API helps websites load faster by letting developers control which resources (like images, scripts, and styles) load first. This improves key performance metrics like Largest Contentful Paint (LCP) and Cumulative Layout Shift (CLS), making pages more user-friendly and efficient.

Key Points:

What it does: Lets you assign priorities (high, low, auto) to resources, ensuring critical content loads first.
Why it matters: Improves speed, user experience, and even conversion rates for key pages.
How to use it: Add fetchpriority attributes in HTML or use JavaScript to dynamically adjust priorities.
Browser support: Fully supported in modern browsers like Chrome, Edge, Firefox, Safari, and Opera.

Quick Example:

<img src="hero.jpg" fetchpriority="high" alt="Hero image">
<link rel="stylesheet" href="critical.css" fetchpriority="high">
<script src="core.js" fetchpriority="high"></script>

By combining Fetch Priority with other tools like lazy loading, preload hints, and responsive images, you can optimize your website’s performance even further.

Fetch Priority API

Resource Loading Basics

How Browsers Load Resources

Browsers prioritize loading key resources like HTML, CSS, and JavaScript before handling secondary assets such as images or background media. Here’s how it works:

Critical Path Resources:
- HTML for parsing and building the DOM
- CSS files needed for rendering the page
- JavaScript files that could block rendering
- Web fonts required for displaying text
Secondary Resources:
- Images that appear below the fold
- Background media files
- Non-essential scripts
- Third-party widgets

This process is designed to optimize the Critical Rendering Path, ensuring users can view content as quickly as possible. Modern browsers also use preload scanners to fetch important resources early, though this approach might not work for every site. Let’s dive into common issues that arise when these patterns aren’t properly optimized.

Common Loading Problems

Performance hiccups can negatively impact user experience. Tools like the Fetch Priority API can address these challenges. Here are some frequent issues:

Resource Contention: When multiple assets compete for limited bandwidth, critical content can be delayed. For example, a large hero image might push back the loading of above-the-fold content.
Render-Blocking Resources: Improperly placed CSS or JavaScript files can block rendering, slowing down metrics like First Contentful Paint (FCP).
Inefficient Resource Order: Browsers sometimes load resources in a less-than-ideal sequence, such as loading non-essential images before critical above-the-fold elements. This can cause layout shifts.

"After OneNine took over one of my client’s website portfolios, we’ve seen each site’s speed increase by over 700%. Load times are now around a second." – Ernest Chapman

Here’s a breakdown of resource types, their default priorities, and common issues:

Resource Type	Default Browser Priority	Common Issues
CSS Files	Highest	Can block rendering if not optimized
JavaScript	High	May block parsing and execution
Images	Low/Medium	Often compete with critical resources
Fonts	Medium	Can cause Flash of Unstyled Text (FOUT)
Third-party Scripts	Low	May delay important content loading

Setting Up Fetch Priority API

HTML Priority Attributes

The Fetch Priority API helps manage resource loading more efficiently with the fetchpriority HTML attribute. This attribute can take values like "high", "low", and "auto". Here are some examples:

Images:
<img src="hero.jpg" fetchpriority="high" alt="Hero image">
<img src="footer-logo.png" fetchpriority="low" alt="Footer logo">

Stylesheets:
<link rel="stylesheet" href="critical.css" fetchpriority="high">
<link rel="stylesheet" href="non-critical.css" fetchpriority="low">

Scripts:
<script src="core.js" fetchpriority="high"></script>
<script src="analytics.js" fetchpriority="low"></script>

Choose the priority value based on the resource’s role. For example, assign "high" to crucial elements like hero images, primary CSS files, and core scripts. Use "low" for less important resources, such as decorative images, non-essential stylesheets, or secondary scripts.

sbb-itb-608da6a

Optimization Tips

Selecting Priority Levels

When using the Fetch Priority API, it’s important to prioritize resources wisely to improve performance. Focus on the elements that shape the user’s first impression:

High-Priority Resources:

Hero images that appear above the fold
Main navigation links
Scripts for core functionality
CSS needed for layout and initial rendering
Key product images on e-commerce sites

Low-Priority Resources:

Images below the fold
Social media widgets
Analytics scripts
Background images
Resources for comment sections
Footer content

For dynamic content, assign high priority to elements that are immediately visible and low priority to others. A good rule of thumb is to aim for about 30% high-priority and 70% low-priority resources. This balance helps browsers manage resources efficiently.

Once priorities are set, combine them with other speed optimization techniques for even better results.

Priority and Other Speed Tools

The Fetch Priority API becomes even more effective when paired with other performance strategies:

Techniques to Combine with Fetch Priority:

Use fetchpriority with lazy loading for images below the fold.
Add preload hints for high-priority resources.
Include resource hints like preconnect and dns-prefetch for critical third-party assets.
Use responsive images and assign appropriate fetchpriority values.
Leverage HTTP/2 server push to deliver critical assets quickly.

Steps for Better Implementation:

Optimize Resources: Compress images and minify code before applying fetchpriority.
Loading Strategy: Use the following HTML for critical assets:

<img 
  src="hero.webp" 
  fetchpriority="high" 
  loading="eager" 
  decoding="async" 
  alt="Hero banner"
>

Monitor Performance: Keep an eye on key metrics like:
- Largest Contentful Paint (LCP)
- First Input Delay (FID)
- Cumulative Layout Shift (CLS)

Use browser developer tools to check if resources load as intended. Pay close attention to network waterfall charts to ensure that high-priority items aren’t delayed by lower-priority ones.

Testing Results

Speed Metrics

Evaluate performance by focusing on key metrics to ensure resources are being prioritized effectively:

Core Web Vitals Metrics:

Largest Contentful Paint (LCP): Tracks how quickly the most important content appears on the screen.
First Input Delay (FID): Measures how fast the site responds to user actions.
Cumulative Layout Shift (CLS): Assesses how stable the page layout is during loading.

Resource Loading Metrics:

Time to First Byte (TTFB): Time it takes for the first byte of the page to load.
First Contentful Paint (FCP): Tracks when the first piece of content is visible.
Time to Interactive (TTI): Measures when the page becomes fully interactive.
Resource load completion times and network waterfall timing to trace resource loading behavior.

Once changes are made, monitor these metrics over 2–3 weeks to account for variations in traffic and caching.

Testing Tools

Compare baseline and post-implementation metrics to measure progress.

Browser Developer Tools:

Chrome DevTools Network Panel
Firefox Performance Tools
Safari Web Inspector

Performance Monitoring Platforms:

Chrome User Experience Report
WebPageTest
Lighthouse
PageSpeed Insights

Establish Baseline Metrics: Gather initial data across different devices and connection speeds to create a performance benchmark.
Monitor Implementation Impact: Track how load times and resource prioritization evolve. Ernest Chapman shared his experience with OneNine, saying:

"After OneNine took over one of my client’s website portfolios, we’ve seen each site’s speed increase by over 700%. Load times are now around a second."
Analyze Resource Loading: Use browser tools to ensure:
- Key resources load first.
- Resources load in the correct sequence based on priority.
- There are no conflicts in resource prioritization.
- fetchpriority attributes are implemented correctly.

Conclusion

Key Takeaways

The Fetch Priority API is a game-changer for improving how resources load on a webpage. Here’s why it matters:

Precise Control: Developers can assign loading priorities to critical resources like hero images, fonts, and scripts.
Boosted Performance: When implemented correctly, this API can improve Core Web Vitals metrics such as Largest Contentful Paint (LCP) and First Input Delay (FID).
Modern Browser Support: Most up-to-date browsers support the API via HTML attributes and JavaScript.
Better Resource Management: Helps avoid conflicts and ensures resources load in the best possible order.

To fully leverage these benefits, websites need expert management and ongoing optimization.

OneNine Services

OneNine

OneNine provides professional services to maximize the benefits of the Fetch Priority API. Their approach focuses on improving website performance through thoughtful resource prioritization and hands-on management.

Clients have shared notable success stories. Here’s what OneNine delivers:

Optimizing performance by prioritizing critical resources
Monitoring Core Web Vitals to ensure consistent improvements
Implementing fetchpriority attributes with technical precision

"OneNine offers outstanding website management with a focus on efficiency and attention to detail. Their timely responses and precision lead to high-quality results, allowing us to concentrate on our key operations."

Achieving better performance isn’t just about technical know-how – it requires ongoing effort and monitoring. With OneNine’s expertise, websites can see measurable improvements in user experience and business outcomes.