css: Removing "shadows" from covered borders - css

Question

I'm trying to show a part of a semicircle with CSS and HTML but there is always a little "shadow" that persists around the covered border. This is my codepen that show what I'm saying. The red on the right shouldn't be shown. Maybe this is a bug of the browser. I tried it on Firefox and Chrome.
This is the sample code:
CSS:
.background {
width: 100%;
height: 500px;
border: 1px solid black;
background-color: black;
}
.circle {
position: absolute;
top: 50px;
left: 50px;
width: 200px;
height: 200px;
border-radius: 50%;
border: 3px solid black;
border-bottom: 3px solid red;
transform: rotate(-20deg);
z-index: 0;
}
.cover {
position: absolute;
top: 50px;
left: 50px;
width: 200px;
height: 200px;
border-radius: 50%;
border: 3px solid black;
border-bottom: 3px solid transparent;
transform: rotate(20deg);
z-index: 10;
}
HTML:
<div class="background">
<div class="cover"></div>
<div class="circle"></div>
</div>

Answer 1

It doesn't seem like you need those two transforms. Try this:
.background {
width: 100%;
height: 500px;
border: 1px solid black;
background: black;
}
.circle {
position: absolute;
top: 50px;
left: 50px;
width: 200px;
height: 200px;
border-radius: 50%;
border: 3px solid black;
border-bottom: 3px solid red;
/* transform: rotate(-20deg); */
z-index: 0;
}
.cover {
position: absolute;
top: 50px;
left: 50px;
width: 200px;
height: 200px;
border-radius: 50%;
border: 3px solid black;
border-bottom: 3px solid transparent;
/* transform: rotate(20deg); */
z-index: 10;
}