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<!DOCTYPE html>
<head>
<title>Images with loading='lazy' load only when in the viewport</title>
<link rel="author" title="Dom Farolino" href="mailto:dom@chromium.org">
<link rel="author" title="Scott Little" href="mailto:sclittle@chromium.org">
<link rel="help" href="https://github.com/scott-little/lazyload">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
</head>
<script>
const in_viewport_test =
async_test("In-viewport loading=lazy images load immediately but do not " +
"block the window load event");
const below_viewport_test =
async_test("Below-viewport loading=lazy images only load when in the " +
"viewport and do not block the window load event");
const below_viewport_data_url_test =
async_test("Below-viewport data:url images only load when in the " +
"viewport and do not block the window load event");
const below_viewport_blob_url_test =
async_test("Below-viewport blob URL images only load when in the " +
"viewport and do not block the window load event");
document.addEventListener('DOMContentLoaded', e => {
const img = document.querySelector('#below_viewport_blob_url');
// Blob URL helper.
// Source: https://bl.ocks.org/nolanlawson/0eac306e4dac2114c752.
function fixBinary(bin) {
const length = bin.length;
const buf = new ArrayBuffer(length);
const arr = new Uint8Array(buf);
for (var i = 0; i < length; i++) {
arr[i] = bin.charCodeAt(i);
}
return buf;
}
const base64 =
"R0lGODlhDAAMAKIFAF5LAP/zxAAAANyuAP/gaP///wAAAAAAACH5BAEAAAUALAAAAAAMAAwAAAMlWLPcGjDKFYi9lxKBOaGcF35DhWHamZUW0K4mAbiwWtuf0uxFAgA";
const binary = fixBinary(atob(base64));
const blob = new Blob([binary], {type: 'image/png'});
const url = URL.createObjectURL(blob);
img.src = url;
}) // DOMContentLoaded.
let has_window_load_fired = false;
let has_in_viewport_loaded = false;
window.onload = e => {
has_window_load_fired = true;
}
// Helper assertion messages for the below tests.
const kScrollAssertion = "images only load when scrolled into view";
const kWindowLoadAssertion = "images do not block the load event";
const in_viewport_img_onload = in_viewport_test.step_func_done(() => {
has_in_viewport_loaded = true;
document.querySelector('#bottom').scrollIntoView();
assert_true(has_window_load_fired,
"In-viewport loading=lazy images do not block the window " +
"load event");
});
const below_viewport_img_onload = below_viewport_test.step_func_done(() => {
assert_true(has_in_viewport_loaded,
"Below-viewport loading=lazy images only load once loaded " +
"into the viewport");
assert_true(has_window_load_fired,
"Below-viewport loading=lazy images should not block the " +
"window load event");
});
const below_viewport_data_url_img_onload = below_viewport_data_url_test.step_func_done(() => {
assert_true(has_in_viewport_loaded,
"Below-viewport loading=lazy data: url images " +
kScrollAssertion);
assert_true(has_window_load_fired,
"Below-viewport loading=lazy data: url images " +
kWindowLoadAssertion);
});
const below_viewport_blob_url_img_onload = below_viewport_blob_url_test.step_func_done(() => {
assert_true(has_in_viewport_loaded,
"Below-viewport loading=lazy blob url images " +
kScrollAssertion);
assert_true(has_window_load_fired,
"Below-viewport loading=lazy blob url images " +
kWindowLoadAssertion);
});
</script>
<body>
<!-- |in_viewport| takes 2 seconds to load, so that in browsers that don't
support lazy loading, |below_viewport| finishes before |in_viewport|, and
the test will dependably fail without relying on a timeout. -->
<img id="in_viewport" loading="lazy" src="resources/image.png?image-loading-lazy-first&pipe=trickle(d2)"
onload="in_viewport_img_onload()">
<div style="height:1000vh;"></div>
<img id="below_viewport" loading="lazy" src="resources/image.png?image-loading-lazy-second"
onload="below_viewport_img_onload()">
<img id="below_viewport_data_url" loading="lazy"
src="data:image/png;base64,R0lGODlhDAAMAKIFAF5LAP/zxAAAANyuAP/gaP///wAAAAAAACH5BAEAAAUALAAAAAAMAAwAAAMlWLPcGjDKFYi9lxKBOaGcF35DhWHamZUW0K4mAbiwWtuf0uxFAgA"
onload="below_viewport_data_url_img_onload()">
<!-- This image has its `src` set to a blob URL dynamically above -->
<img id="below_viewport_blob_url" loading="lazy"
onload="below_viewport_blob_url_img_onload()">
<div id="bottom"></div>
</body>
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