CSS实现盒子水平垂直居中的方法

	<div class="outside" id="outside">
        <div class="inside">嘿嘿嘿</div>
    </div>

• 利用定位，已知盒子的宽高

        .outside {
            width: 800px;
            height: 600px;
            border: 1px solid red;
            margin: 100px auto;
            position: relative;
        }

        .inside {
            width: 200px;
            height: 100px;
            background-color: gold;
            position: absolute;
            top: 50%;
            left: 50%;
            margin-top: -50px;
            margin-left: -100px;
        }

• 利用定位，盒子的宽高动态变化

  		.outside {
            width: 800px;
            height: 600px;
            border: 1px solid red;
            margin: 100px auto;
            position: relative;
        }
        
        .inside {
            width: 20%;
            height: 10%;
            background-color: gold;
            position: absolute;
            top: 0;
            left: 0;
            bottom: 0;
            right: 0;
            margin: auto;
        }

• 利用定位和translate，不考虑盒子宽高

		.outside {
            width: 800px;
            height: 600px;
            border: 1px solid red;
            margin: 100px auto;
            position: relative;
        }
        
        .inside {
            background-color: gold;
            position: absolute;
            top: 50%;
            left: 50%;
            transform: translate(-50%,-50%)
        }

• 利用flex实现

		.outside{
            width: 800px;
            height: 600px;
            border: 1px solid red;
            margin: 100px auto;
            display: flex;
            justify-content: center;
            align-items: center;
        }
        .inside {
            border: 1px dotted blue;
            padding: 5px;
            display: inline;
        }

• 利用js来实现

		.body{
            position: relative;
        }
        .outside{
            width: 60px;
            height: 30px;
            border: 1px solid red;
            margin: 100px auto;
        }

<script>
    let winW = document.documentElement.clientWidth;
    let winH = document.documentElement.clientHeight;
    let boxW = outside.offsetWidth
    let boxH = outside.offsetHeight
    outside.style.position = "absolute"
    outside.style.left = (winW-boxW) / 2 + "px"
    outside.style.top = (winH-boxH) / 2 + "px"
</script>

• 利用display: table-cell;

		/* 必须固定宽高 */
		 .outside{
            width: 800px;
            height: 600px;
            border: 1px solid red;
            margin: 100px auto;
            display: table-cell;
            vertical-align: middle;
            text-align: center;

        }
        .inside {
            border: 1px dotted blue;
            padding: 5px;
            display: inline-block
        }