css how to make a box centered with position code example

Example 1: center a div in css

.container {
  display: flex;
  justify-content: center;
  align-items: center;
}

Example 2: 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 */