How can I center a div within another div? [duplicate]

Answer 1

* {
  box-sizing: border-box;
}

#main_content {
  top: 160px;
  left: 160px;
  width: 800px;
  min-height: 500px;
  height: auto;
  background-color: #2185c5;
  position: relative;
}

#container {
  width: 50%;
  height: 50%;
  margin: auto;
  padding: 10px;
  position: absolute;
  border: 5px solid yellow;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
}