css水平垂直居中

1. flex

.parent{
    height: 500px;
    width: 500px;
    border: 1px solid red;
    display: flex;
    align-items: center;
    justify-content: center;
}
.child{
    height: 80px;
    width: 80px;
    background-color: #bbb;
}

2. tabel-cell

.parent{
    height: 500px;
    width: 500px;
    border: 1px solid red;
    display: table-cell;
    text-align: center;
    vertical-align: middle;
}
.child{
    display: inline-block;
    height: 80px;
    width: 80px;
    background-color: #bbb;
}

3. 定位 + transform

.parent{
    height: 500px;
    width: 500px;
    border: 1px solid red;
    position: relative;
}
.child{
    height: 80px;
    width: 80px;
    background-color: #bbb;
    position: absolute;
    left: 50%;
    top: 50%;
    transform: translate(-50%, -50%);
}

4. margin + transform

.parent{
    height: 500px;
    width: 500px;
    border: 1px solid red;
}
.child{
    height: 80px;
    width: 80px;
    background-color: #bbb;
    margin: 50% 0 0 50%;
    transform: translate(-50%, -50%);
}

5. 定位 + 负margin

.parent{
    height: 500px;
    width: 500px;
    border: 1px solid red;
    position: relative;
}
.child{
    height: 80px;
    width: 80px;
    background-color: #bbb;
    position: absolute;
    left: 50%;
    top: 50%;
    margin: -40px 0 0 -40px;
}

参考资料