FlexBox:

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

Grid:

.container{
    display:grid;
    place-items:center;
}

Position:

.container{
    position:relative;
}
.box{
    position:absolute;
    inset:0;
    margin:auto
}

Margin:

.container{
    display:grid;
}
.box{   
    margin:auto
}