It seems Apple's overlay is more than just a transparency. How can I achieve this effect with CSS and possibly JavaScript?
It is possible with CSS 3:
#myDiv {
-webkit-filter: blur(20px);
-moz-filter: blur(20px);
-o-filter: blur(20px);
-ms-filter: blur(20px);
filter: blur(20px);
opacity: 0.4;
}
An example is here: JSFiddle