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Test Info:

<meta charset=utf-8>
<title>Largest Contentful Paint: observe text.</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
p {
font-size: 12px;
async_test(function (t) {
assert_implements(window.LargestContentfulPaint, "LargestContentfulPaint is not implemented");
let beforeRender;
const observer = new PerformanceObserver(
t.step_func_done(function(entryList) {
assert_equals(entryList.getEntries().length, 1);
const entry = entryList.getEntries()[0];
assert_equals(entry.entryType, 'largest-contentful-paint');
assert_greater_than_equal(entry.renderTime, beforeRender);
assert_greater_than_equal(, entry.renderTime);
assert_approx_equals(entry.startTime, entry.renderTime, 0.001,
'startTime should be equal to renderTime to the precision of 1 millisecond.');
assert_equals(entry.duration, 0);
// Some lower bound: height of at least 12 px.
// Width of at least 100 px.
// TODO: find a good way to bound text width.
assert_greater_than_equal(entry.size, 1200);
assert_equals(entry.loadTime, 0);
assert_equals(, 'my_text');
assert_equals(entry.url, '');
assert_equals(entry.element, document.getElementById('my_text'));
observer.observe({type: 'largest-contentful-paint', buffered: true});
beforeRender =;
}, 'Text element is observable as a LargestContentfulPaint candidate.');
<p id='my_text'>This is important text! :)</p>