html position absolute center
snippet in html

position absolute center

user471

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

html position absolute center

user6010

position: absolute;
margin-left: auto;
margin-right: auto;
left: 0;
right: 0;
text-align: center;