前端一些小技巧

<div class="con">
        <img src="left.png" >
</div>    

.con{
           width: 200px;
           height: 200px;
           border: 1px solid #ccc; 
           /* 让图片左右居中 */
           text-align: center; 
           /* 让图片上下垂直居中 */
           vertical-align: middle;
           display: table-cell;
       }

    <div class="con2">
        <img src="left.png" >
    </div>

        .con2{
            width: 200px;
            height: 200px;
            border:1px solid #ccc;
            position: relative;
        }
        .con2 img{
            position: absolute;
            left:0;
            top:0;
            bottom:0;
            right:0;
            margin: auto;
        }

    <div class="con3">
        <img src="left.png" >
    </div>

        .con3{
            width: 200px;
            height: 200px;
            border:1px solid #ccc;
            position: relative;
        }
        .con3 img{
            position: absolute;
            left:50%;
            top:50%;
            transform: translate(-50%,-50%);
        }

        .con4 {
            display:box;
            display:-webkit-box;
            -webkit-box-align: center;
            -webkit-box-pack: center;
        }

    <div>
        <p class="wen">
            <span>你是谁为了谁我的箱底姐妹，啦啦啦，黑猫警长，不要问我从哪里来我的故乡在运你是谁为了谁我的箱底姐妹，啦啦啦，黑猫警长，不要问我从哪里来我的故乡在运放放</span>
        </p>
    </div>

 .wen{
            width: 400px;
            height: 200px;
            border: 1px solid red;
            font-size: 20px;
            display: table-cell;
            vertical-align: middle;
        }