.box {
      background: lightgreen;
      border: 50px solid green;
      margin: 1em;
    }

    #box1 {
      box-sizing: content-box; /*default*/
      width: 300px;
      height: 300px;
      padding: 20px;
    }

    #box2 {
      box-sizing: border-box;
      width: 300px;
      height: 300px;  
      padding: 20px;
    }