SOURCE

<div class="container">
    <span class="btn">click me</span>
    <p>测试测试测试</p>
    <div>
        <span class="pop">Hello World</span>
    </div>
</div>

.container {
    padding: 100px;
    height: 2000px;
    margin: 100px;
    line-height: 100px;
}

.btn {
    anchor-name: --anchor-el;
}

@position-try --bottom {
  position-area: bottom;
}

.pop {
    position-anchor: --anchor-el;
    position: absolute;
    position-area: top;
    /* bottom: anchor(top); */
    /* position-try-fallbacks: --bottom; */
    position-try-fallbacks: flip-block;
}