<!DOCTYPE html>
<style>
#scroll-container {
width: 400px;
height: 400px;
overflow: scroll;
}
#scroll-contents {
width: 1000px;
height: 1000px;
position: relative;
}
#anchor {
anchor-name: --anchor;
height: 100px;
width: 100px;
overflow: scroll;
}
#anchored {
background: green;
width: 100px;
height: 100px;
}
</style>
<div id="scroll-container">
<div id="scroll-contents">
<div style="height: 400px"></div>
<div id="anchored"></div>
<div id="anchor">
<div style="height: 500px"></div>
</div>
</div>
</div>
<script>
document.getElementById('scroll-container').scrollTop = 300;
document.getElementById('anchor').scrollTop = 300;
</script>