A close button X with a single div element

single-element-x.css

body{
    background:#333;
}

div{
    width:40px;
    height:40px;
    background-color:#333;
    position:relative;
    border-radius:6px;
    box-shadow:2px 2px 4px 0 white;
}

div:before,div:after{
    content:'';
    position:absolute;
    width:36px;
    height:4px;
    background-color:white;
    border-radius:2px;
    top:16px;
    box-shadow:0 0 2px 0 #ccc;
}

div:before{
    -webkit-transform:rotate(45deg);
    -moz-transform:rotate(45deg);
    transform:rotate(45deg);
    left:2px;
}
div:after{
    -webkit-transform:rotate(-45deg);
    -moz-transform:rotate(-45deg);
    transform:rotate(-45deg);
    right:2px;
}

single-element-x.html

<div></div>