html - Why do I get a faint border around the border of a rotated div in CSS?

Question

I am doing a this challenge on CSS Battle and get a very thin border around the rotated div object. Why is that? How can I get rid of it? When I submit it on the website it also scores only 98.somewhat %, so it's not just a rendering problem.

<div id="a"></div>
<div id="b"></div>
<div id="c"></div>
<style>
  body {
    background: #222730;
    margin: 0;
  }
  
  div {
    position: absolute;
    background: #4CAAB3;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
  }
  
  #a {
    width: 400px;
    height: 150px;
  }
  
  #b {
    z-index: 1;
    border: solid 50px #222730;
    width: 150px;
    height: 150px;
    transform: translate(-50%, -50%) rotate(45deg);
  }
  
  #c {
    z-index: 2;
    background: #393E46;
    border-radius: 25px;
    width: 50px;
    height: 50px;
  }
</style>

question from:https://stackoverflow.com/questions/65909176/why-do-i-get-a-faint-border-around-the-border-of-a-rotated-div-in-css

Answer 1

It's coming from the background property of #b (inherited from div).

Simply shift this property setting to be exclusive to #a:

body {
  background: #222730;
  margin: 0;
}

div {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

#a {
  width: 400px;
  height: 150px;
  background: #4CAAB3;
}

#b {
  z-index: 1;
  border: solid 50px #222730;
  width: 150px;
  height: 150px;
  transform: translate(-50%, -50%) rotate(45deg);
}

#c {
  z-index: 2;
  background: #393E46;
  border-radius: 25px;
  width: 50px;
  height: 50px;
}

<div id="a"></div>
<div id="b"></div>
<div id="c"></div>