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Debug Web Vitals in the field

Debug Web Vitals in the field

Learn how to attribute your Web Vitals data with debug information
to help you identify and fix real-user issues with analytics

Appears in: Web Vitals

Google currently provides two categories of tools to measure and debug Web Vitals:

  • Lab tools: Tools such as Lighthouse, where your page is loaded in a simulated environment that can mimic various conditions (for example, a slow network and a low-end mobile device).
  • Field tools: Tools such as Chrome User Experience Report (CrUX), which is based on aggregate, real-user data from Chrome. (Note that the field data reported by tools such as PageSpeed Insights and Search Console is sourced from CrUX data.)

While field tools offer more accurate data—data which actually represents what real users experience—lab tools are often better at helping you identify and fix issues.

CrUX data is more representative of your page's real performance, but knowing your CrUX scores is unlikely to help you figure out how to improve your performance.

Lighthouse, on the other hand, will identify issues and make specific suggestions for how to improve. However, Lighthouse will only make suggestions for performance issues it discovers at page load time. It does not detect issues that only manifest as a result of user interaction such as scrolling or clicking buttons on the page.

This raises an important question: how can you capture debug information for the Web Vitals metric data from real users in the field?

This post will explain in detail what APIs you can use to collect additional debugging information for each of the current Core Web Vitals metrics and give you ideas for how to capture this data in your existing analytics tool.

APIs for attribution and debugging #


Of all the Core Web Vitals metrics, CLS is perhaps the one for which collecting debug information in the field is the most important. CLS is measured throughout the entire lifespan of the page, so the way a user interacts with the page—how far they scroll, what they click on, and so on—can have a significant impact on whether there are layout shifts and which elements are shifting.

Consider the following report from PageSpeed Insights for the URL:

A PageSpeed Insights Report with different CLS values

The value reported for CLS from the lab (Lighthouse) compared to the CLS from the field (CrUX data) are quite different, and this makes sense if you consider that the page has a lot of interactive content that is not being used when tested in Lighthouse.

But even if you understand that user interaction affects field data, you still need to know what elements on the page are shifting to result in a score of 0.45 at the 75th percentile.

The LayoutShiftAttribution interface makes that possible.

Get layout shift attribution #

The LayoutShiftAttribution interface is exposed on each layout-shift entry that Layout Instability API emmits.

For a detailed explanation of both of these interfaces, see Debug layout shifts. For the purposes of this post, the main thing you need to know is that, as a developer, you are able to observe every layout shift that happens on the page as well as what elements are shifting.

Here's some example code that logs each layout shift as well as the elements that shifted:

new PerformanceObserver((list) => {
for (const {value, startTime, sources} of list.getEntries()) {
// Log the shift amount and other entry info.
console.log('Layout shift:', {value, startTime});
if (sources) {
for (const {node, curRect, prevRect} of sources) {
// Log the elements that shifted.
console.log(' Shift source:', node, {curRect, prevRect});
}).observe({type: 'layout-shift', buffered: true});

It's probably not practical to measure and send data to your analytics tool for every single layout shift that occurs; however, by monitoring all shifts, you can keep track of the worst shifts and just report information about those.

The goal isn't to identify and fix every single layout shift that occurs for every user, the goal is to identify the shifts that affect the largest number of users and thus contribute the most to your page's CLS at the 75th percentile.

Also, you don't need to compute the largest source element every time there's a shift, you only need to do so when you're ready to send the CLS value to your analytics tool.

The following code takes a list of layout-shift entries that have contributed to CLS and returns the largest source element from the largest shift:

function getCLSDebugTarget(entries) {
const largestEntry = entries.reduce((a, b) => {
return a && a.value > b.value ? a : b;
if (largestEntry && largestEntry.sources && largestEntry.sources.length) {
const largestSource = largestEntry.sources.reduce((a, b) => {
return a.node && a.previousRect.width * a.previousRect.height >
b.previousRect.width * b.previousRect.height ? a : b;
if (largestSource) {
return largestSource.node;

Once you've identified the largest element contributing to the largest shift, you can report that to your analytics tool.

The element contributing the most to CLS for a given page will likely vary from user to user, but if you aggregate those elements across all users, you'll be able to generate a list of shifting elements affecting the most number of users.

Once you've identified and fixed the root cause of the shifts for those elements, your analytics code will start reporting smaller shifts as the "worst" shifts for your pages. Eventually, all reported shifts will be small enough that your pages are well within the "good" threshold of 0.1!

Some other metadata that may be useful to capture along with the largest shift source element is:

  • The time of the largest shift
  • The URL path at the time of the largest shift (for sites that dynamically update the URL, such as Single Page Applications).


To debug LCP in the field, the primary information you need is which particular element was the largest element (the LCP candidate element) for that particular page load.

Note that it's entirely possible—in fact, it's quite common—that the LCP candidate element will be different from user to user, even for the exact same page.

This can happen for several reasons:

  • User devices have different screen resolutions, which results in different page layouts and thus different elements being visible within the viewport.
  • Users don't always load pages scrolled to the very top. Oftentimes links will contain fragment identifiers or even text fragments, which means it's possible for your pages to be loaded and displayed at any scroll position on the page.
  • Content may be personalized for the current user, so the LCP candidate element could vary wildly from user to user.

This means you cannot make assumptions about which element or set of elements will be the most common LCP candidate element for a particular page. You have to measure it based on real-user behavior.

Identify the LCP candidate element #

To determine the LCP candidate element in JavaScript you can use the Largest Contentful Paint API, the same API you use to determine the LCP time value.

Given a list of largest-contentful-paint entries, you can determine the current LCP candidate element by looking at the last entry:

function getLCPDebugTarget(entries) {
const lastEntry = entries[entries.length - 1];
return lastEntry.element;

Caution: As explained in the LCP metric documentation, the LCP candidate element can change through the page load, so more work is required to identify the "final" LCP candidate element. The easiest way to identify and measure the "final" LCP candidate element is to use the web-vitals JavaScript library, as shown in the example below.

Once you know the LCP candidate element, you can send it to your analytics tool along with the metric value. As with CLS, this will help you identify which elements are most important to optimize first.

Some other metadata that may be useful to capture along with the LCP candidate element:

  • The image source URL (if the LCP candidate element is an image).
  • The text font family (if the LCP candidate element is text and the page uses web fonts).


To debug FID in the field, it's important to remember that FID measures only the delay portion of the overall first input event latency. That means that what the user interacted with is not really as important as what else was happening on the main thread at the time they interacted.

For example, many JavaScript applications that support server-side rendering (SSR) will deliver static HTML that can be rendered to the screen before it's interactive to user input—that is, before the JavaScript required to make the content interactive has finished loading.

For these types of applications, it can be very important to know whether the first input occurred before or after hydration. If it turns out that many people are attempting to interact with the page before hydration completes, consider rendering your pages in a disabled or loading state rather than in a state that looks interactive.

If your application framework exposes the hydration timestamp, you can easily compare that with the timestamp of the first-input entry to determine whether the first input happened before or after hydration. If your framework doesn't expose that timestamp, or doesn't use hydration at all, another useful signal may be whether input occurred before or after JavaScript finished loading.

The DOMContentLoaded event fires after the page's HTML has completely loaded and parsed, which includes waiting for any synchronous, deferred, or module scripts (including all statically imported modules) to load. So you can use the timing of that event and compare it to when FID occurred.

The following code takes a first-input entry and returns true if the first input occurred prior to the end of the DOMContentLoaded event:

function wasFIDBeforeDCL(fidEntry) {
const navEntry = performance.getEntriesByType('navigation')[0];
return navEntry && fidEntry.startTime < navEntry.domContentLoadedEventStart;

If your page uses async scripts or dynamic import() to load JavaScript, the DOMContentLoaded event may not be a useful signal. Instead, you can consider using the load event or—if there's a particular script you know takes a while to execute—you can use the Resource Timing entry for that script.

Identify the FID target element #

Another potentially useful debug signal is the element that was interacted with. While the interaction with the element itself does not contribute to FID (remember FID is just the delay portion of the total event latency), knowing which elements your users are interacting with may be useful in determining how best to improve FID.

For example, if the vast majority of your user's first interactions are with a particular element, it consider inlining the JavaScript code needed for that element in the HTML, and lazy loading the rest.

To get the element associated with the first input event, you can reference the first-input entry's target property:

function getFIDDebugTarget(entries) {
return entries[0].target;

Some other metadata that may be useful to capture along with the FID target element:

  • The type of event (such as mousedown, keydown, pointerdown).
  • Any relevant long task attribution data for the long task that occurred at the same time as the first input (useful if the page loads third-party scripts).

Usage with the web-vitals JavaScript library #

The sections above offer some suggestions for additional debug info to include in the data you send to your analytics tool. Each of the examples includes some code that uses one or more performance entries associated with a particular Web Vitals metric and returns a DOM element that can be used to help debug issues affecting that metric.

These examples are designed to work well with the web-vitals JavaScript library, which exposes the list of performance entries on the Metric object passed to each callback function.

If you combine the examples listed above with the web-vitals metric functions, the end result will look something like this:

import {getLCP, getFID, getCLS} from 'web-vitals';

function getSelector(node, maxLen = 100) {
let sel = '';
try {
while (node && node.nodeType !== 9) {
const part = ? '#' + : node.nodeName.toLowerCase() + (
(node.className && node.className.length) ?
'.' + Array.from(node.classList.values()).join('.') : '');
if (sel.length + part.length > maxLen - 1) return sel || part;
sel = sel ? part + '>' + sel : part;
if ( break;
node = node.parentNode;
} catch (err) {
// Do nothing...
return sel;

function getLargestLayoutShiftEntry(entries) {
return entries.reduce((a, b) => a && a.value > b.value ? a : b);

function getLargestLayoutShiftSource(sources) {
return sources.reduce((a, b) => {
return a.node && a.previousRect.width * a.previousRect.height >
b.previousRect.width * b.previousRect.height ? a : b;

function wasFIDBeforeDCL(fidEntry) {
const navEntry = performance.getEntriesByType('navigation')[0];
return navEntry && fidEntry.startTime < navEntry.domContentLoadedEventStart;

function getDebugInfo(name, entries = []) {
// In some cases there won't be any entries (e.g. if CLS is 0,
// or for LCP after a bfcache restore), so we have to check first.
if (entries.length) {
if (name === 'LCP') {
const lastEntry = entries[entries.length - 1];
return {
debug_target: getSelector(lastEntry.element),
event_time: lastEntry.startTime,
} else if (name === 'FID') {
const firstEntry = entries[0];
return {
debug_target: getSelector(,
debug_timing: wasFIDBeforeDCL(firstEntry) ? 'pre_dcl' : 'post_dcl',
event_time: firstEntry.startTime,
} else if (name === 'CLS') {
const largestEntry = getLargestLayoutShiftEntry(entries);
if (largestEntry && largestEntry.sources && largestEntry.sources.length) {
const largestSource = getLargestLayoutShiftSource(largestEntry.sources);
if (largestSource) {
return {
debug_target: getSelector(largestSource.node),
event_time: largestEntry.startTime,
// Return default/empty params in case there are no entries.
return {
debug_target: '(not set)',

function sendToAnalytics({name, value, entries}) {
navigator.sendBeacon('/analytics', JSON.stringify({
...getDebugInfo(name, entries)


The specific format required to send the data will vary by analytics tool, but the above code should be sufficient to get the data needed, regardless of the format requirements.

The code above also includes a getSelector() function (not mentioned in previous sections), which takes a DOM node and returns a CSS selector representing that node and its place in the DOM. It also takes an optional maximum length parameter (defaulting to 100 characters) in the event that your analytics provider has length restrictions on the data you send it.

Report and visualize the data #

Once you've started collecting debug information along with your Web Vitals metrics, the next step is aggregating the data across all your users to start looking for patterns and trends.

As mentioned above, you don't necessarily need to address every single issue your users are encountering, you want to address—especially at first—the issues that are affecting the largest number of users, which should also be the issues that have the largest negative impact on your Core Web Vitals scores.

The Web Vitals Report tool #

If you're using the Web Vitals Report tool, it's been recently updated to support reporting on a single debug dimension for each of the Core Web Vitals metrics.

Here's a screenshot from the Web Vitals Report debug info section, showing data for the Web Vitals Report tool website itself:

Web Vitals Report showing debug information

Using the data above, you can see that whatever is causing the section.Intro element to shift is contributing the most to CLS on this page, so identifying and fixing the cause of that shift will yield the greatest improvement to the score.

Summary #

Hopefully this post has helped outline the specific ways you can use the existing performance APIs to get debug information for each of the Core Web Vitals metrics based on real-user interactions in the field. While it's focused on the Core Web Vitals, the concepts also apply to debugging any performance metric that's measurable in JavaScript.

If you're just getting started measuring performance, and you're already a Google Analytics user, the Web Vitals Report tool may be a good place to start because it already supports reporting debug information for each of the Core Web Vitals metrics.

If you're an analytics vendor and you're looking to improve your products and provide more debugging information to your users, consider some of the techniques described here but don't limit yourself to just the ideas presented here. This post is intended to be generally applicable to all analytics tools; however, individual analytics tools likely can (and should) capture and report even more debug information.

Lastly, if you feel there are gaps in your ability to debug these metrics due to missing features or information in the APIs themselves send your feedback to

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