My div doesn't rotate at the center of the screen

Question

It is my first question because I can't find solution for my easy problem (i guess). I don't know why my div is changing its position when it should rotate around itself. How can i resolve a problem of this type? I found some similar questions but about different location of their divs that don't point me to the answer.

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  font-family: sans-serif;
  font-size: 30px;
  color: white;
  background-color: #333;
  overflow: hidden;
}

.square {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  height: 400px;
  width: 400px;
  background-color: gold;
  border: 3px solid black;
  animation: square 4s ease-in-out .5s infinite;
  animation-direction: alternate;
  transition: transform 2s;
}

@keyframes square {
  from {
    top: 20%;
    background-color: aquamarine;
    transform: rotate(0deg);
  }
  to {
    top: 80%;
    background-color: sandybrown;
    transform: rotate(360deg);
  }
}

<div class="square"></div>

Answer 1

You need to add 3 things:

1.) Add transform-origin: left top; to .square(because that "neutralizes your -50% top and left shifting)

2.+3.) Add translate(-50%, -50%) to the two keyframe stages to make the object stay at the original transform setting, otherwise transform will switch to the default 0/0 values

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  font-family: sans-serif;
  font-size: 30px;
  color: white;
  background-color: #333;
  overflow: hidden;
}

.square {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  height: 400px;
  width: 400px;
  background-color: gold;
  border: 3px solid black;
  animation: square 4s ease-in-out .5s infinite;
  animation-direction: alternate;
  transition: transform 2s;
  transform-origin: left top;
}

@keyframes square {
  from {
    background-color: aquamarine;
    transform: rotate(0deg) translate(-50%, -50%);
  }
  to {
    background-color: sandybrown;
    transform: rotate(360deg) translate(-50%, -50%);
  }
}

<div class="square"></div>