CSS子元素在父元素中水平垂直居中的几种方法

        #div1{
            width: 300px;
            height: 300px;
            border: 1px solid red;
        }
        #div1 p {
            width: 100px;
            height: 100px;
            background-color: green;

            /*float: left;    !*如果设置了浮动，则自动居中就会失效。*!*/
            margin: 0 auto;
        }

        <div id="div1">
            <p></p>
        </div>

       #div2 {
            width: 300px;
            height: 300px;
            border: 1px solid red;

            /*position: relative;*/
            text-align: center;
        }
        #div2 p{
            width: 100px;
            height: 100px;
            background-color: green;
            margin: 0;

            /*float: left;     !*如果设置了浮动，则自动居中就会失效。*!*/
            display: inline-block;
            /*display: inline;*/
            /*display: inline-flex;*/
        }    

        <div id="div2">
            <h4>其他内容</h4>
            <p></p>
            <h3>其他内容</h3>
        </div>    

        #div3 {
            width: 300px;
            height: 300px;
            border: 1px solid red;

            position: relative;
        }
        #div3 p {
            width: 100px;
            height: 100px;
            background-color: green;

            /*margin-top: 10%;      !*百分比相对于父元素*!*/
            /*padding-top: 10%;        !*百分比相对于父元素*!*/
            position: absolute;
            top: 50%;
            left: 50%;
            margin-top: -50px;
            margin-left: -50px;
        }

        <div id="div3">
            <p></p>
            <h3>其他内容</h3>
        </div>

       #div4{
            width: 300px;
            height: 300px;
            border: 1px solid red;

            position: relative;
        }
        #div4 p{
            width: 100px;
            height: 100px;
            background-color: green;

            position: absolute;
            top: 0;
            left: 0;
            bottom: 0;
            right: 0;
            margin: auto;
        }

        <div id="div4">
            <p></p>
            <h3>其他内容</h3>
        </div>

        #div5{
            width: 300px;
            height: 300px;
            border: 1px solid red;

            display: table-cell;
            vertical-align: middle;
        }
        #div5 p{
            width: 100px;
            height: 100px;
            background-color: green;

            margin: auto;
        }

        <div id="div5">
            <p></p>
        </div>

        #div6{
            width: 300px;
            height: 300px;
            border: 1px solid red;
        }
        #div6 p {
            width: 100px;
            height: 100px;
            background-color: green;
            margin: 0;

            position: relative;
            top: 50%;
            left: 50%;
            transform: translate(-50%, -50%);
        }

        <div id="div6">
            <p></p>
            <h3>其他内容</h3>
        </div>

        #div7{
            width: 300px;
            height: 300px;
            border: 1px solid red;

            position: relative;
        }
        #div7 p {
            width: 100px;
            height: 100px;
            background-color: green;
            margin: 0;

            position: absolute;
            top: 50%;
            left: 50%;
            transform: translate(-50%, -50%);
        }

        <div id="div7">
            <p></p>
            <h3>其他内容</h3>
        </div>

       #div8{
            width: 300px;
            height: 300px;
            border: 1px solid red;

            display: flex;
            justify-content: center;
            align-items: center;
        }
        #div8 p{
            width: 100px;
            height: 100px;
            background-color: green;
            margin: 0;
        }

        <div id="div8">
            <p></p>
        </div>