fix a div center of page even if another div add or removes on top code example

Example 1: center with css

.parent {
  position: relative;
}
.child {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

Example 2: how to put container in center of page

#container
{
    width: 400px;
    margin-left: auto;
    margin-right: auto;
}