how to align an html page in the middle code example

Example 1: center div horizontally and vertically

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

Example 2: html center body

body {
  display:flex; flex-direction:column; justify-content:center;
  min-height:100vh;
}

Example 3: html center body

body {
max-width: max-content;
margin: auto;
}