Six ways:
Absolute plus margin solution fixed plus margin solution display:table solution line-height solution flex layout solution transform unknown element width and height solution

Absolute + margin scheme

    div{
        position: absolute;
        width: 100px;
        height: 100px;
        left: 50%;
        top: 50%:
        margin-top: -50px;
        margin-left: -50px;
    }

Fixed plus margin scheme

    div{
        position: fixed;
        width: 100px;
        height: 100px;
        top: 0;
        right:0;
        left: 0;
        bottom: 0;
        margin: auto;
    }

Display: table scheme

    div{
        display: table-cell;
        vertical-align: middle;
        text-align: center;
        width: 100px;
        height: 100px;
    }

The line-height scheme for the inline element

    div{
        text-align: center;
        line-height: 100px;
    }

Flex Flexible Layout Solution

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

Transform unknown element width and height solution

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