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    <div class="caja caja-sombra sombra1 sombra2">
           <h2>Ejemplo 5</h2>
          <p>Sombras laterales interiores</p>
        </div>

    CSS

    .caja{
        
        width: 200px;
        height: 150px;
        background-color: #f4f4f4;
       border-radius: 5px; 
      border: 1px dotted #9C9C9C;
    }
    .caja-sombra{
        position: relative;
        float: left;
        padding: 1em;
        margin: 100px 50px;
        box-shadow: 0 1px 4px rgba(0, 0, 0, 0.3), 0 0 40px rgba(0, 0, 0, 0.1) inset;
    }
    .caja-sombra:before, .caja-sombra:after{
        content: "";
        position: absolute;
        z-index: -2;
    }
    .sombra1:beforez{
        top: 10px;
        bottom: 10px;
        left: 0;
        right: 50%;
        box-shadow: 0 0 15px rgba(0,0,0,0.6);
        border-radius: 5px / 100px;
    }
    .sombra2:before{
        right: 0
    }