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<!DOCTYPE html>
<title>CSS Anchor Positioning: basic last successful position fallback on pseudo-elements</title>
<link rel="author" title="Kiet Ho" href="mailto:kiet.ho@apple.com">
<link rel="help" href="https://drafts.csswg.org/css-anchor-position/#last-successful-position-fallback">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="support/test-common.js"></script>
<style>
#container {
position: relative;
width: 400px;
height: 400px;
background: teal;
}
#anchor {
position: relative;
top: 100px;
left: 100px;
width: 100px;
height: 100px;
background: red;
anchor-name: --a;
}
#anchored::before {
position-anchor: --a;
/*
We can't use position-area, since there's no way to access offsetTop of
pseudo-elements to check for position. Instead, we have to use anchor()
and check the computed inset values.
*/
bottom: anchor(top);
left: anchor(left);
position-try-fallbacks: flip-block;
position: absolute;
width: 100px;
height: 200px;
background: lime;
content: "";
}
</style>
<div id="container">
<div id="anchor"></div>
<div id="anchored"></div>
</div>
<script>
promise_test(async () => {
await waitUntilNextAnimationFrame();
await waitUntilNextAnimationFrame();
const anchoredStyle = getComputedStyle(anchored, "::before");
assert_equals(anchoredStyle.top, "200px");
}, "Starts rendering with flip-block");
promise_test(async () => {
anchor.style.top = "150px";
await waitUntilNextAnimationFrame();
await waitUntilNextAnimationFrame();
const anchoredStyle = getComputedStyle(anchored, "::before");
assert_equals(anchoredStyle.top, "250px");
}, "No successful position, keep flip-block");
promise_test(async () => {
anchor.style.top = "200px";
await waitUntilNextAnimationFrame();
await waitUntilNextAnimationFrame();
const anchoredStyle = getComputedStyle(anchored, "::before");
assert_equals(anchoredStyle.top, "0px");
}, "Base position without fallback now successful");
</script>
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