Optimize resource loading with the Fetch Priority API

Addy Osmani

Leena Sohoni

Patrick Meenan

Barry Pollard

Browser Support

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When a browser parses a web page and begins to discover and download resources such as images, scripts, or CSS, it assigns them a fetch priority so it can download them in an optimal order. A resource's priority usually depends on what it is and where it is in the document. For example, in-viewport images might have a High priority, and the priority for early loaded, render-blocking CSS using <link>s in the <head> might be Very High. Automatic priority assignment usually works well, but there are use cases where the assigned order isn't optimal.

This page discusses the Fetch Priority API and the fetchpriority HTML attribute, which help you optimize the Core Web Vitals by letting you hint at the relative priority of a resource (high or low).

Summary

A few key areas where Fetch Priority can help:

Boosting the priority of the LCP image by specifying fetchpriority="high" on the image element, causing LCP to happen sooner.
Increasing the priority of async scripts, using better semantics than the current most common hack (inserting a <link rel="preload"> for the async script).
Decreasing the priority of late-body scripts to sequence images better.

A filmstrip
view comparing two tests of the Google Flights homepage. At the bottom,
Fetch Priority is used to boost the priority of the hero image, resulting in
a 0.7 second decrease in LCP. — Fetch Priority improving Largest Contentful Paint from 2.6 s to 1.9 s in a test of Google Flights.

Historically, developers have had limited influence over resource priority using preload and preconnect. Preload lets you tell the browser about critical resources you want to load early before the browser would naturally discover them. This is especially useful for resources that are harder to discover, such as fonts included in stylesheets, background images, or resources loaded from a script. Preconnect helps warm up connections to cross-origin servers and can help improve metrics like Time to first byte. It's useful when you know an origin but not necessarily the exact URL of a resource that will be needed.

Fetch Priority complements these Resource Hints. It's a markup-based signal available through the fetchpriority attribute that developers can use to indicate the relative priority of a particular resource. You can also use these hints through JavaScript and the Fetch API with the priority property to influence the priority of resource fetches made for data. Fetch Priority can also complement preload. Take a Largest Contentful Paint image, which, when preloaded, will still get a low priority. If it is pushed back by other early low-priority resources, using Fetch Priority can help how soon the image gets loaded.

Resource priority

The resource download sequence depends on the browser's assigned priority for every resource on the page. The factors that can affect priority computation logic include the following:

The type of resource, such as CSS, fonts, scripts, images, and third-party resources.
The location or order the document references resources in.
The preload resource hint, which helps the browser to discover a resource faster and load it earlier.
Priority computation changes for async or defer scripts.

The following table shows how Chrome prioritizes and sequences most resources:

	Load in layout-blocking phase		Load one-at-a-time in layout-blocking phase
Blink Priority	VeryHigh	High	Medium	Low	VeryLow
DevTools Priority	Highest	High	Medium	Low	Lowest
	Main resource
	CSS (early**)		CSS (late**)		CSS (media mismatch***)
		Script (early** or not from preload scanner)	Script (late**)	Script (async)
	Font	Font (rel=preload)
		Import
		Image (in viewport)	Image (first 5 images > 10,000px2)	Image
				Media (video/audio)
					Prefetch
		XSL
	XHR (sync)	XHR/fetch* (async)

Notes:
* Preloads using "as" or fetch with a specific "destination" use the priority of the destination they're requesting. For example, preload as="style" uses Highest priority. With no explicit destination, fetch is prioritized like an async XHR.
** "Early" is defined as being requested before any non-preloaded images have been requested, and "late" is after non-preloaded images have been requested.
*** CSS where the media type doesn't match isn't fetched by the preload scanner and is only processed when the main parser reaches it, which usually means it's fetched very late.

Images start at "Low" priority by default. At layout time, images within the viewport are boosted to "High". As of Chrome 117, the first 5 large images are set to "Medium" to speed this up, and two of them can be fetched in parallel during the initial "tight mode". By tagging important images in markup using fetchpriority="high", they can start at "High" immediately and load much faster.

The browser downloads resources with the same computed priority in the order they're discovered. You can check the priority assigned to different resources when loading a page under the Chrome Dev Tools Network tab. (Make sure you include the priority column by right-clicking the table headings).

A screenshot of assets listed in the network tab of Chrome's DevTools. The columns read, from left to right: name, status, type, initiator, size, time, and priority. — Priority for resource `type = "font"` on BBC news detail page

When priorities change, you can see both the initial and final priority in the Big request rows setting or in a tooltip.

A screenshot of assets listed in the network tab of Chrome's DevTools. The 'Big request rows' setting is ticked and the Priority column shows the first image with a prioruty of High, and a different initial priority of medium below. The same is shown in the tooltip. — Priority changes in DevTools.

When might you need Fetch Priority?

Now that you understand the browser's prioritization logic, you can adjust your page's download order to optimize its performance and Core Web Vitals. Here are some examples of things you can change without using Fetch Priority:

Place resource tags like <script> and <link> in the order you want the browser to download them.
Use the preload resource hint to download necessary resources earlier, especially resources that are harder for the browser to discover.
Use async or defer to download scripts without blocking other resources.
Lazy-load below-the-fold content so the browser can use the available bandwidth for more critical above-the-fold resources.

Here are some more complex cases where Fetch Priority can help you get the resource priority order you need:

You have several above-the-fold images, but not all of them should have the same priority. For example, in an image carousel, only the first visible image needs a higher priority.
Hero images inside the viewport typically start at a Low or Medium priority. After the layout is complete, Chrome discovers that they're in the viewport and boosts their priority. This usually adds a significant delay to loading the images. Providing the Fetch Priority in markup lets the image start at a "High" priority and start loading much earlier.

Preload is still required for early discovery of LCP images included as CSS backgrounds. To boost your background images' priority, include fetchpriority='high' on the preload.
Declaring scripts as async or defer tells the browser to load them asynchronously. However, as shown in the priority table, these scripts are also assigned a "Low" priority. You might want to increase their priority while ensuring asynchronous download, especially for scripts that are critical for the user experience.
If you use the JavaScript fetch() API to fetch resources or data asynchronously, the browser assigns it High priority. You might want some of your fetches to run with lower priority, especially if you're mixing background API calls with API calls that respond to user input. Mark the background API calls as Low priority and the interactive API calls as High priority.
The browser assigns CSS and fonts a High" priority, but some of those resources might be more important than others. You can use Fetch Priority to lower the priority of non-critical resources.

The `fetchpriority` attribute

Use the fetchpriority HTML attribute to specify download priority for resource types such as CSS, fonts, scripts, and images when downloaded using link, img, or script tags. It can take the following values:

high: The resource is a high priority, and you want the browser to prioritize it as long as the browser's own heuristics don't prevent that from happening.
low: The resource is a low priority, and you want the browser to deprioritize it if its heuristics let it.
auto: The default value, which lets the browser choose the appropriate priority.

Here are a few examples of using the fetchpriority attribute in markup, as well as the script-equivalent priority property.

<!-- We don't want a high priority for this above-the-fold image -->
<img src="/images/in_viewport_but_not_important.svg" fetchpriority="low" alt="I'm an unimportant image!">

<!-- We want to initiate an early fetch for a resource, but also deprioritize it -->
<link rel="preload" href="/js/script.js" as="script" fetchpriority="low">

<script>
  fetch('https://example.com/', {priority: 'low'})
  .then(data => {
    // Trigger a low priority fetch
  });
</script>

Effects of browser priority and `fetchpriority`

You can apply the fetchpriority attribute to different resources as shown in the following table to increase or reduce their computed priority. fetchpriority="auto" (◉) in each row marks the default priority for that type of resource.

	Load in layout-blocking phase		Load one at a time in layout-blocking phase
Blink Priority	VeryHigh	High	Medium	Low	VeryLow
DevTools Priority	Highest	High	Medium	Low	Lowest
Main Resource	◉
CSS (early**)	⬆◉	⬇
CSS (late**)		⬆	◉	⬇
CSS (media mismatch***)		⬆***			◉⬇
Script (early** or not from preload scanner)		⬆◉		⬇
Script (late**)		⬆	◉	⬇
Script (async/defer)		⬆		◉⬇
Font	◉
Font (rel=preload)		⬆◉		⬇
Import		◉
Image (in viewport - after layout)		⬆◉		⬇
Image (first 5 images > 10,000px2)		⬆	◉	⬇
Image		⬆		◉⬇
Media (video/audio)				◉
XHR (sync) - deprecated	◉
XHR/fetch* (async)		⬆◉		⬇
Prefetch					◉
XSL		◉

fetchpriority sets relative priority, meaning it raises or lowers the default priority by an appropriate amount, rather instead of explicitly setting the priority to High or Low. This often results in High or Low priority, but not always. For example, critical CSS with fetchpriority="high" retains the "VeryHigh"/"Highest" priority, and using fetchpriority="low" on these elements retains the "High" priority. Neither of these cases involve explicitly setting priority to High or Low.

Use cases

Use the fetchpriority attribute when you want to give the browser an extra hint about what priority to fetch a resource with.

Increase the priority of the LCP image

You can specify fetchpriority="high" to boost the priority of the LCP or other critical images.

<img src="lcp-image.jpg" fetchpriority="high">

The following comparison shows the Google Flights page with an LCP background image loaded with and without Fetch Priority. With the priority set to high, the LCP improved from 2.6s to 1.9s.

An experiment conducted using Cloudflare workers to rewrite the Google Flights page using Fetch Priority.

Lower the priority of above-the-fold images

Use fetchpriority="low" to lower the priority of above-the-fold images that aren't immediately important, for example in an image carousel.

<ul class="carousel">
  <img src="img/carousel-1.jpg" fetchpriority="high">
  <img src="img/carousel-2.jpg" fetchpriority="low">
  <img src="img/carousel-3.jpg" fetchpriority="low">
  <img src="img/carousel-4.jpg" fetchpriority="low">
</ul>

In an earlier experiment with the Oodle app, we used this to lower the priority of images that don't appear on load. It decreased page load time by 2 seconds.

A side-by-side comparison of Fetch Priority when used on the Oodle app's image carousel. On the left, the browser sets default priorities for carousel images, but downloads and paints those images around two seconds slower than the example on the right, which sets a higher priority on only the first carousel image. — Using high priority for only the first carousel image lets the page load faster.

Lower the priority of preloaded resources

To stop preloaded resources from competing with other critical resources, you can reduce their priority. Use this technique with images, scripts, and CSS.

<!-- Lower priority only for non-critical preloaded scripts -->
<link rel="preload" as="script" href="critical-script.js">
<link rel="preload" href="/js/script.js" as="script" fetchpriority="low">

<!-- Preload CSS without blocking other resources -->
<link rel="preload" as="style" href="theme.css" fetchpriority="low" onload="this.rel='stylesheet'">

Reprioritize scripts

Scripts your page needs to be interactive have to load quickly, but shouldn't block other resources. You can mark these as async with high priority.

<script src="async_but_important.js" async fetchpriority="high"></script>

You can't mark a script as async if it relies on specific DOM states. However, if they run later on the page, you can load them with lower priority:

<script src="blocking_but_unimportant.js" fetchpriority="low"></script>

Lower the priority for non-critical data fetches

The browser executes fetch with a high priority. If you have multiple fetches that might fire simultaneously, you can use the high default priority for the more important data fetches and lower the priority of less critical data.

// Important validation data (high by default)
let authenticate = await fetch('/user');

// Less important content data (suggested low)
let suggestedContent = await fetch('/content/suggested', {priority: 'low'});

Fetch Priority implementation notes

Here are some things to be aware of when using Fetch Priority:

The fetchpriority attribute is a hint, not a directive. The browser tries to respect the developer's preference, but it can also apply its resource priority preferences for resource priority to resolve conflicts.
Don't confuse Fetch Priority with preloading:
- Preload is a mandatory fetch, not a hint.
- Preload lets the browser discover a resource early, but it still fetches the resource with the default priority. Meanwhile, Fetch Priority doesn't help with discoverability, but it does let you increase or decrease the fetch priority.
- It's easier to observe and measure the effects of a preload than the effects of a priority change.
Fetch Priority can complement preloads by increasing the granularity of prioritization. If you've already specified a preload as one of the first items in the <head> for an LCP image, then a high Fetch Priority might not improve LCP significantly. However, if the preload happens after other resources load, a high Fetch Priority can improve LCP more. If a critical image is a CSS background image, preload it with fetchpriority = "high".
Load time improvements from prioritization are more relevant in environments where more resources compete for the available network bandwidth. This is common for HTTP/1.x connections where parallel downloads aren't possible, or on low bandwidth HTTP/2 connections. In these cases, prioritization can help resolve bottlenecks.
CDNs don't implement HTTP/2 prioritization uniformly. Even if the browser communicates the priority from Fetch Priority, the CDN might not reprioritize resources in the specified order. This makes testing Fetch Priority difficult. The priorities are applied both internally within the browser and with protocols that support prioritization (HTTP/2 and HTTP/3). It's still worth using Fetch Priority just for the internal browser prioritization independent of CDN or origin support, because priorities often change when the browser requests resources. For example, low-priority resources like images are often held back from being requested while the browser processes critical <head> items.
You might not be able to introduce Fetch Priority as a best practice in your initial design. Later in your development cycle, you can the priorities being assigned to different resources on the page, and if they don't match your expectations, you can introduce Fetch Priority for further optimization.

Tips for using preloads

Keep the following in mind when using preloads:

Including a preload in HTTP headers puts it before everything else in the load order.
Generally, preloads load in the order the parser gets to them for anything above "Medium" priority. Be careful if you're including preloads at the beginning of your HTML.
Font preloads probably work best toward the end of the head or beginning of the body.
Import preloads (dynamic import() or modulepreload) should run after the script tag that needs the import, so make sure the script gets loaded or parsed first so it can be evaluated while its dependencies are loading.
Image preloads have a "Low" or "Medium" priority by default. Order them relative to async scripts and other low or lowest priority tags.

History

Fetch Priority was first experimented with in Chrome as an origin trial in 2018, and then again in 2021 using the importance attribute. At that time it was called Priority Hints. The interface has since changed to fetchpriority for HTML and priority for JavaScript's Fetch API as part of the web standards process. To reduce confusion, we now call this API Fetch Priority.