块级元素的三种垂直水平居中的方法

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

.parent{
    width: 500px;
    height: 500px;
    margin: 0 auto;
    border: 1px solid gainsboro;
    display: flex;
    justify-content: center;
    align-items: center;
}
.child{
    width: 200px;
    height: 200px;
    background: red;
}

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

.parent{
        position: relative;
        width: 500px;
        height: 500px;
        margin: 0 auto;
        border: 1px solid gainsboro;
    }
.child{
        position: absolute;
        : 50%;
        left: 50%;
        width: 200px;
        height: 200px;
        margin-left: -100px;
        margin-: -100px;
        background: red;
    }

<div class="parent">
    <div class="child">
        <span>我是子元素</span>
    </div>
</div>

.parent{
        position: relative;
        width: 500px;
        height: 500px;
        margin: 0 auto;
        border: 1px solid gainsboro;
    }
    .child{
        position: absolute;
        : 50%;
        left: 50%;
        transform: translate(-50%,-50%);
        background: red;
    }