三栏布局，div左右盒子是定宽，中间是自适应

用弹性布局flex:

给父盒子加个display:flex; 给中间盒子设flex=1;

   /* 弹性盒子布局*/
        .wrap{
             100%;
            height: 90px;
            display: flex;
        }
        .left{
             300px;
            height: 90px;
            background-color: red;
            float: left;
        }
        .content{
            flex:1;
            height: 90px;
            background-color: yellow;
        }
        .right{
             300px;
            height: 90px;
            background-color:blue;
            float: right;
        }
    </style>
</head>
<body>
<div class="wrap">
    <div class="left"></div>
    <div class="content"></div>
    <div class="right"></div>
</div>