how to place something in the middle of the div in css' code example

Example 1: css center text in div

/* For horizontal align: */
parent-element {text-align:center;}
/* For horizontal and vertical align: */
parent-element {position: relative;}
element-to-be-centered {
  position: absolute;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
}

/* See https://www.w3schools.com/css/css_align.asp for more info */

Example 2: how to put container in center of page

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