top 50% and translateY(-50%) not working as expected

I am trying to vertically align the div using the age old technique of

top: 50%; transform: translateY(-50%);

but it is not working as I expect it to. I am wondering what the problem here could be.

I tried adding display flex to the body and adding align-content: center; but that just jumbles everything. So I tried the aforementioned technique but it's just placing the div halfway up the screen without the top: 50%; affecting it at all.

Here's the code:

body {
  margin: 0;
  padding: 0;
  overflow: hidden;
}

#mittrek {
  width: 80%;
  max-width: 200px;
  height: 400px;
  border-radius: 20px;
  z-index: 10;
  background-color: rgba(0, 0, 0, 0.5);
  position: relative;
  margin-left: auto;
  margin-right: auto;
  top: 50%;
  transform: translateY(-50%);
  backdrop-filter: blur(15px);
  -webkit-backdrop-filter: blur(10px);
  background-repeat: no-repeat;
  display: flex;
  align-content: center;
  justify-content: center;
}

#mittrek::after {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  padding: 4px;
  border-radius: 20px;
  mask-composite: exclude;
  z-index: 4;
  overflow: hidden;
  box-shadow: 0px 0px 40px 15px black;
}

<!DOCTYPE html>

<html>
<body>
  <div id="mittrek"></div>
</body>
</html>

Solution

You set wrong value of position property for mittrek. This is updated code.

<!DOCTYPE html>

<html>
<head>
  <style>
    body {
      margin: 0;
      padding: 0;
      overflow: hidden;
    }

    #mittrek {
      width: 80%;
      max-width: 200px;
      height: 400px;
      border-radius: 20px;
      z-index: 10;
      background-color: rgba(0, 0, 0, 0.5);
      position: absolute; // Changed relative to absolute
      margin-left: auto;
      margin-right: auto;
      top: 50%;
      left: 50%; // Added left
      transform: translate(-50%, -50%); // Changed translateY to translate
      backdrop-filter: blur(15px);
      -webkit-backdrop-filter: blur(10px);
      background-repeat: no-repeat;
      display: flex;
      align-content: center;
      justify-content: center;
    }

    #mittrek::after {
      content: '';
      position: absolute;
      top: 0;
      left: 0;
      right: 0;
      bottom: 0;
      padding: 4px;
      border-radius: 20px;
      mask-composite: exclude;
      z-index: 4;
      overflow: hidden;
      box-shadow: 0px 0px 40px 15px black;
    }
  </style>
</head>
<body>
  <div id="mittrek"></div>
</body>
</html>