<style>
    div {
    width: 50px;
    height: 50px;
    float: left;
    margin: 0 25px 50px 25px;
    background-color: lightyellow;
}
</style>
<!-- Positive spread does not increase border radii. -->
<div style="
    -webkit-box-shadow: black 0 100px 0 20px;
    -webkit-border-radius: 10px;
"></div>
<!-- It uses the actual radii (in this case, 25px after scaling
     to fit in a total side length of 50px) not the specified radii. -->
<div style="
    -webkit-box-shadow: black 0 100px 0 20px;
    -webkit-border-radius: 50px;
"></div>
<!-- Negative spread decreases the radii. -->
<div style="
    -webkit-box-shadow: black 0 100px 0 -10px;
    -webkit-border-radius: 20px;
"></div>