<div class="parent">
    parent
    <br>
    <div class="child">
        child
    </div>
</div>

html, body {
    margin: 0;
    width: 100%;
    height: 100%;

    .parent {
        width: 100%;
        height: 100%;
        background: #fac3fa;
        
        .child {
            width: 50%;
            height: 50%;
            background: #fe9d38;
        }
    }
}

.parent {
    text-align: center;

    .child {
        display: inline-block;
    }
}

.child {
    display: block; // 非块级元素时设置
    margin: 0 auto;
}

.parent {
    position: absolute;
    left: 50%;
    transform: translateX(-50%);
}

.parent {
    display: flex;
    justify-content: center;
}

<div class="parent">
    <div class="child"></div>
</div>

html, body {
    margin: 0;
    width: 100%;
    height: 100%;
}
.parent {
    display: table-cell;
    width: 800px;
    height: 500px;
    background: #fac3fa;

    .child {
        width: 50%;
        height: 50%;
        background: #fe9d38;
    }
}

.parent {
    display: table-cell;
    vertical-align: middle;
}

.parent {
    position: relative;
    
    .child {
        position: absolute;
        top: 50%;
        transform: translateY(-50%);
    }
}

.parent {
    display: flex;
    align-items: center;
}

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

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