Aligning horizontally and vertically a div with absolute position and unknown size inside a div

When the size of the div with absolute position is unknown, the simplest solution is using translate function. Obviously, the container is not static.

.absolute1 {
    position:absolute;
    background-color: antiquewhite;     
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Element with absolute position and unknown size

Element with absolute position and unknown size

Element with absolute position and unknown size

By the way, many authors suggest a solution that works only for divs with height and width set:

.absolute2{
    position:absolute;
    background-color: antiquewhite;    
    top: 0; 
    left: 0; 
    bottom: 0; 
    right: 0;
    margin: auto;
    width:50%;
    height:30%;
}

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Relative Container, Relative Container, Relative Container, Relative Container, Relative Container

Element with absolute position and known size

Element with absolute position and known size

Element with absolute position and known size