#div_1{
    border: 1px solid black;
    background: lightgray;
    opacity:.5;
    height:200px;
    width:300px;
    float:left;
}

#div_2{
    border: 1px solid black;
    background: yellow;
    opacity:.5;
    height:300px; /*auto;*/
    width:200px;
    /*float:right;*/
}
.clear_float{
    border: 1px solid black;
    clear: both;    
}

#div_3{
    
    border: 1px solid black;
    background: orange;
    opacity:.5;
    height:300px;
    width:300px;
}