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Largest Contentful Paint
Making it easier to know when a page's important content has loaded.
philipwalton
2019-08-08
hero.jpg
top
Artist painting tools
Use the Largest Contentful Paint API to optimize for faster page loads.
post
performance
metrics

Historically, it's been a challenge for web developers to measure how quickly the main content of a web page loads and is visible to users.

Older metrics like load or DOMContentLoaded are not good because they don't necessarily correspond to what the user sees on their screen. And newer, user-centric performance metrics like First Paint (FP) and First Contentful Paint (FCP) only capture the very beginning of the loading experience. If a page shows a splash screen or displays a loading indicator, this moment is not very relevant to the user.

In the past we've recommended performance metrics like First Meaningful Paint (FMP) and Speed Index (SI) (both available in Lighthouse) to help capture more of the loading experience after the initial paint, but these metrics are complex, hard to explain, and often wrong—meaning they still do not identify when the main content of the page has loaded.

Sometimes simpler is better. Based on discussions in the W3C Web Performance Working Group and research done at Google, we've found that a more accurate way to measure when the main content of a page is loaded is to look at when the largest element was rendered.

Largest Contentful Paint defined

The Largest Contentful Paint (LCP) API, available in Chrome 77, reports the render time of the largest content element visible in the viewport.

What elements are considered?

As currently specified, the types of elements considered for Largest Contentful Paint are:

  • <img> elements
  • <image> elements inside an <svg> element
  • <video> elements (the poster image is used)
  • An element with a background image loaded via the url() function (as opposed to a CSS gradient)
  • Block-level elements containing text nodes or other inline-level text elements children.

Note, restricting the elements to this limited set was intentional in order to keep things simple in the beginning. Additional elements (e.g. <svg>, <video>) may be added in the future as more research is conducted.

How is an element's size determined?

The size of the element reported for Largest Contentful Paint is typically the size that's visible to the user within the viewport. If the element extends outside of the viewport, or if any of the element is clipped or has non-visible overflow, those portions do not count toward the element's size.

For image elements that have been resized from their intrinsic size, the size that gets reported is either the visible size or the intrinsic size, whichever is smaller. For example, images that are shrunk down to a much smaller than their intrinsic size will only report the size they're displayed at, whereas images that are stretched or expanded to a larger size will only report their intrinsic sizes.

For text elements, only the size of their text nodes is considered (the smallest rectangle that encompases all text nodes).

For all elements, any margin, padding, or border applied via CSS is not considered.

{% Aside %} Determining which text nodes belong to which elements can sometimes be tricky, especially for elements whose children includes inline elements and plain text nodes but also block-level elements. The key point is that every text node belongs to (and only to) its closest block-level ancestor element. In spec terms: each text node belongs to the element that generates its containing block. {% endAside %}

When is largest contentful paint reported?

Web pages often load in stages, and as a result, it's possible that the largest element on the page might change.

To handle this potential for change, the browser dispatches a PerformanceEntry of type largest-contentful-paint identifying the largest contentful element as soon as the browser has painted the first frame. But then, after rendering subsequent frames, it will dispatch another PerformanceEntry any time the largest contentful element changes.

It's important to note that an element can only be considered the largest contentful element once it has rendered and is visible to the user. Images that have not yet loaded are not considered "rendered". Neither are text nodes using web fonts during the font block period. In such cases a smaller element may be reported as the largest contentful element, but as soon as the larger element finishes rendering, it'll be reported via another PerformanceEntry object.

In addition to late-loading images and fonts, a page may add new elements to the DOM as new content becomes available. If any of these new elements is larger than the previous largest contentful element, a new PerformanceEntry will also be reported.

If a page removes an element from the DOM, that element will no longer be considered. Similarly, if an element's associated image resource changes (e.g. changing img.src via JavaScript), then that element will stop be considered until the new image loads.

The browser will stop reporting new entries as soon as the user interacts with the page (via a tap, scroll, or keypress), as user interaction often changes what's visible to the user (which is especially true with scrolling).

For analysis purposes, you should only report the most recently dispatched PerformanceEntry to your analytics service.

{% Aside 'caution' %} Since users can open pages in a background tab, it's possible that the largest contentful paint will not happen until the user focuses the tab, which can be much later than when they first loaded it. {% endAside %}

Load time vs. render time

For security reasons, the render timestamp of images is not exposed for cross-origin images that lack the Timing-Allow-Origin header. Instead, only their load time is exposed (since this is already exposed via many other web APIs).

The usage example below shows how to handle elements whose render time is not available. But, when possible, it's always recommended to set the Timing-Allow-Origin header, so your metrics will be more accurate.

How are element layout and size changes handled?

To keep the performance overhead of calculating and dispatching new performance entries low, changes to an element's size or position do not generate new LCP candidates. Only the element's initial size and position in the viewport is considered.

This means images that are initially rendered off-screen and then transition on-screen may not be reported. It also means elements initially rendered in the viewport that then get pushed down, out of view will still report their initial, in-viewport size.

However, (as mentioned above) an element will be removed from consideration if it's removed from the DOM or if its associated image resource changes.

Examples

Here are some examples of when the Largest Contentful Paint occurs on a few popular websites:

Largest Contentful Paint timeline from cnn.com

Largest Contentful Paint timeline from techcrunch.com

In both of the timelines above, the largest element changes as content loads. In the first example, new content is added to the DOM and that changes what element is the largest. In the second example, the layout changes and content that was previously the largest is removed from the viewport.

While it's often the case that late-loading content is larger than content already on the page, that's not necessarily the case. The next two examples show the Largest Contentful Paint occurring before the page fully loads.

Largest Contentful Paint timeline from instagram.com

Largest Contentful Paint timeline from google.com

In the first example, the Instagram logo is loaded relatively early and it remains the largest element even as other content is progressively shown. In the Google search results page example, the largest element is a paragraph of text that is displayed before any of the images or logo finish loading. Since all the individual images are smaller than this paragraph, it remains the largest element throughout the load process.

{% Aside %} In the first frame of the Instagram timeline, you may notice the camera logo does not have a green box around it. That's because it's an <svg> element, and <svg> elements are not currently considered LCP candidates. The first LCP candidate is the text in the second frame. {% endAside %}

How to measure Largest Contentful Paint in JavaScript

The following code block shows how to measure LCP in JavaScript and report it to an analytics service when the user leaves the tab:

// Create a variable to hold the latest LCP value (since it can change).
let lcp;

// Create the PerformanceObserver instance.
const po = new PerformanceObserver((entryList) => {
  const entries = entryList.getEntries();
  const lastEntry = entries[entries.length - 1];

  // Update `lcp` to the latest value, using `renderTime` if it's available,
  // otherwise using `loadTime`. (Note: `renderTime` may not be available if
  // the element is an image and it's loaded cross-origin without the
  // `Timing-Allow-Origin` header.)
  lcp = lastEntry.renderTime || lastEntry.loadTime;
});

// Observe entries of type `largest-contentful-paint`, including buffered
// entries, i.e. entries that occurred before calling `observe()`.
po.observe({type: 'largest-contentful-paint', buffered: true});

// Send the latest LCP value to your analytics server once the user
// leaves the tab.
addEventListener('visibilitychange', function fn() {
  if (lcp && document.visibilityState === 'hidden') {
    sendToAnalytics({'largest-contentful-paint': lcp});
    removeEventListener('visibilitychange', fn, true);
  }
}, true);

Note, this example waits until the user leaves the tab to report LCP as a way of ensuring it only reports the latest entry. If you would prefer to report every entry (to avoid potentially missing sessions), make sure to configure your analytics to only include the last entry received per page load.

What if the largest element isn't the most important?

In some cases the most important element (or elements) on the page is not the same as the largest element, and developers may be more interested in measuring the render times of these other elements instead. This is possible using the Element Timing API.

The Largest Contentful Paint API is actually built on top of the Element Timing API and adds automatic reporting of the largest contentful element, but you can report on additional elements by explicitly adding the elementtiming attribute to them, and registering a PerformanceObserver to observe the element entry type.

Here's the example used in the specification:

<img... elementtiming='foobar'/>
<p elementtiming='important-paragraph'>This is text I care about.</p>
...
<script>
const observer = new PerformanceObserver((list) => {
  let perfEntries = list.getEntries();
  // Process the entries by iterating over them.
});
observer.observe({type: 'element', buffered: true});
</script>

{% Aside %} Important: the types of elements considered for Largest Contentful Paint are the same as those observable via the Element Timing API. If you add the elementtiming attribute to an element that isn't one of those types, it will be ignored. {% endAside %}

How to improve Largest Contentful Paint on your site

LCP is primarily affected by three factors:

  • Server response time
  • CSS blocking time
  • Asset/subresource load time

Also, if your site is client-rendered, and your largest contentful elements are added to the DOM via JavaScript, then your script's parse, compile, and execution time can also be a factor in LCP.

Here is some guidance you can reference to optimize all of these factors:

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