Llamar a un div con transiciones mediante css

[IsaBelM]

haz estos cambios
css

.modalmask {
position: absolute;
font-family: Arial, sans-serif;
top: 0;
right: 0;
bottom: 0;
left: 0;
background: rgba(0,0,0,0.8);
z-index: 99999;
opacity:0;
pointer-events: none;
}


.modalmaskshow {
-webkit-transition: opacity 3s ease-in;
-moz-transition: opacity 3s ease-in;
transition: opacity 3s ease-in;
opacity:1;
}

js

function stopscrollhref(){
var ventana = document.getElementById('modal1');
ventana.className = 'modalmaskshow';
}

html

<img alt='' src='http://1.bp.blogspot.com/-E0fmDTqIfEk/VkDog-17ZKI/AAAAAAAAAFY/UHeLmInqaBA/s1600/e-mail.png' style='vertical-align:middle;border:0' onclick='javascript:stopscrollhref()'/>

para efectos algo mas complejos, lee este articulo