<!DOCTYPE HTML>
<p>Below there should be a solid black circle.</p>
<br>
<div style="position:relative; z-index:1; overflow:hidden; margin-top:100px; width:200px; height:200px; border-radius:50%; border:10px solid black; background:black;">
    <div style="transform:translateZ(0); width:300px; height:300px; margin-left:-50px; margin-top:-50px; background:black;"></div>
</div>