Answer by user1432181 for Which font for CSS "x" close button?

Using modern browsers you can rotate a + sign:

.popupClose:before {
    content:'+';
}
.popupClose {
    position:relative;
    float:right;
    right:10px;
    top:0px;
    padding:2px;
    cursor:pointer;
    margin:2px;
    color:grey;
    font-size:32pt;
    transform:rotate(45deg);
}

then simply place the following html where you need:

 <span class='popupClose'></span>