html 结构：

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

css代码：

• 水平居中

1. inline-block + text-align

.parent{
    text-align: center;
}
.child{
    display: inline-block;
}

2. table + margin

.child{
    display: table;
    margin: 0 auto;
}

3. absolute + transform

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

4. flex + justify-content / margin

.parent{
    display: flex;
    justify-content: center;
}
.child{
    margin: 0 auto;
}

• 垂直居中

1. table-cell + vertical-align

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

2. absolute + transform

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

3. flex + align-items

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

• 水平居中 + 垂直居中

1. inline-block + text-align + table-cell + vertical-align

.parent{
    text-align: center;
    display: table-cell;
    vertical-align: middle;
}
.child{
    display: inline-block;
}

2. absolute + transform

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

3. flex + justify-content

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