SOURCE

<div class="container">
    <div class="content"></div>
</div>

<div class="tabel">
  <div>1</div>
  <div>2</div>
</div>

.container{
            margin-top: 100px;
            text-align: center;
            height: 25px;
            background: gray;   
        }
        .content{
            width: 100%;
            height: 100%;
            width: 300px;
            height: 50px;
            background: gray;
            display: inline-block;
            position: relative;
            top: -25px;
        }
        .content::before{
            content: '';
            position: absolute;
            top: 0;
            left: -25px;
            width: 0;
            height: 0;
            border-right: 25px solid #ddd;
            border-top: 25px solid transparent;
            border-bottom: 25px solid transparent;
        }
        .content::after{
            content: '';
            position: absolute;
            top: 0;
            right: -25px;
            width: 0;
            height: 0;
            border-left: 25px solid #ddd;
            border-top: 25px solid transparent;
            border-bottom: 25px solid transparent;
        }

.table{
  display: table;
  table-layout: fixed;
}
.table div{
  display: table-cell;
}