<!DOCTYPE html>

<!--
    The perspective origin should be positioned in the container's border box,
    regardless of the presence of scrollbars.
-->

<style>
.container {
    background-color: green;
    border: 10px solid black;
    overflow: scroll;
    position: relative;
    width: 100px;
    height: 100px;
    margin: 10px 0;
    perspective: 500px;
}
.transformed {
    background-color: blue;
    width: 50px;
    height: 50px;
    transform: translateZ(-1000px);
}
</style>

<div class="container">
    <div class="transformed"></div>
</div>

<!-- This blue box should be smaller because it should be obscured by the border. -->
<div class="container" style="perspective-origin: 0 0">
    <div class="transformed"></div>
</div>

<div class="container" style="perspective-origin: 60% 40px">
    <div class="transformed"></div>
</div>