<!DOCTYPE html>

<meta charset="utf-8">

<link rel="author" href="mailto:zhoupeng.1996@bytedance.com">

<link rel="help" href="https://www.w3.org/TR/css-position-3/#fixed-cb">

<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/12352">

<script src="/resources/testharness.js"></script>

<script src="/resources/testharnessreport.js"></script>

<style>

#a, #b, #c, #d, #e, #f {

position: fixed;

}

#margin {

margin: 10px;

}

#transform {

transform: translateX(10px);

}

#perspective {

perspective: 10px;

}

#filter {

filter: opacity(25%);

}

#contain {

contain: paint;

}

</style>

<body>

<div id="a"></div>

<div id="margin">

<div><div id="b"></div></div>

</div>

<div id="transform">

<div><div id="c"></div></div>

</div>

<div id="perspective">

<div><div id="d"></div></div>

</div>

<div id="filter">

<div><div id="e"></div></div>

</div>

<div id="contain">

<div><div id="f"></div></div>

</div>

</body>

<script>

test(() => {

assert_equals(a.offsetParent, null);

assert_equals(b.offsetParent, null);

}, 'If the containing block for a fixed positioned element is viewport, the offsetParent should be null.');

test(() => {

assert_equals(c.offsetParent, transform);

assert_equals(d.offsetParent, perspective);

assert_equals(e.offsetParent, filter);

assert_equals(f.offsetParent, contain);

}, 'If the containing block for a fixed positioned element is the nearest ancestor box, the offsetParent should be the nearest ancestor box.');

</script>