How to position absolute but relative to parent with CSS?

To position absolute but relative to parent with CSS, we set the position property.

For instance, we write

#father {
  position: relative;
}

#son1 {
  position: absolute;
  top: 100%;
}

to set the parent element to have relative position with position: relative;.

Then we set the child to have absolute position with position: absolute;, which will position it relative to the parent.