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<!DOCTYPE html>
<style>
div {
border: 2px solid black;
padding: 5px 0; /* No horizontal padding as outline-offset is not affected by it and span simulates it */
font-size: 16px;
}
span {
display: block;
/* 3/6px offset for border */
height: 2px;
width: calc(100% - 20em + 6px);
margin: -1px calc(10em - 3px); /* -1px vertical to remove height of span from div height */
background: red;
}
</style>
<p>PASS if there is a thin red line in the middle of the box.</p>
<div>
<span></span>
</div>