If applied CSS is not working, then try to avoid blank spaces in the CSS.

Good CSS:

#popup
{
display:none;
position:absolute;
height:100px;
width:100px;
background:#E6E6FA;     
border: 2px solid black;
overflow:auto;
}


Bad CSS:

#popup
{
    display:none;
    position:absolute;
    height:100px;
    width:100px;
    background:#E6E6FA;     
    border: 2px solid black;
    overflow:auto;
}