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Test Info:

<meta charset=utf-8>
<title>Largest Contentful Paint: largest text is reported.</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style type="text/css">
#text2 {
position: absolute;
width: auto;
white-space: nowrap;
<!-- These are some text and some tiny images. We care about the largest text. -->
<img id='image1' />
<div id='text1'></div>
<div id='text2'></div>
<img id='image2' />
const load_image = async (id, url) => {
await new Promise(resolve => {
const image = document.getElementById(id);
image.addEventListener('load', resolve);
image.src = url;
const load_text = (id, text) => {
let div = document.getElementById(id);
div.innerHTML = text;
promise_test(async (t) => {
assert_implements(window.LargestContentfulPaint, "LargestContentfulPaint is not implemented");
let beforeRender =;
// Load images and add texts.
await load_image('image1', '/images/lcp-1x1.png');
load_text('text1', 'This is some text.');
load_text('text2', 'This is more text so it will be the Largest Contentful Paint!');
await load_image('image2', '/images/lcp-2x2.png');
await new Promise(resolve => {
new PerformanceObserver(
(entryList, observer) => {
entryList.getEntries().forEach(entry => {
// The tiny images or text1 could be reported as LCP if it is rendered before text2.
if ( !== 'text2')
assert_equals(entry.entryType, 'largest-contentful-paint',
'The entry entryType should be largest-contentful-paint.');
assert_greater_than_equal(entry.renderTime, beforeRender,
'The entry renderTime should be greater than or equal to the beforeRender.');
assert_greater_than_equal(, entry.renderTime,
'The timestamp should be greater than or equal to the entry renderTime.');
assert_approx_equals(entry.startTime, entry.renderTime, 0.001,
'The entry startTime should be equal to renderTime to the precision of 1 millisecond.');
assert_equals(entry.duration, 0, 'The entry duration should be 0.');
const div = document.getElementById('text2');
// The div styling makes it approximate the text size.
assert_greater_than_equal(entry.size, (div.clientHeight - 5) * (div.clientWidth - 5),
'Reported LCP size should not be significantly smaller than the text2 div.');
assert_less_than_equal(entry.size, (div.clientHeight + 1) * (div.clientWidth + 1),
'Reported LCP size should not be larger than the text2 div.');
assert_equals(entry.loadTime, 0, 'The entry loadTime should be 0.');
assert_equals(entry.url, '', 'The entry url should be empty.');
assert_equals(entry.element, div, 'The entry element should be test2 div.');
}).observe({ type: 'largest-contentful-paint', buffered: true });
}, 'Largest Contentful Paint: largest text is reported.');