I am laying a rectangular div on top of a down-arrow image so that only a triangular piece peeks out (I did this by setting the rectangle's z-index to 100 and the arrow's z-index to -100). When I add a drop-shadow to the down-arrow image, I expect only the triangular piece's drop-shadow to appear, but the rest of the down-arrow's drop-shadow that should be hidden under the rectangular div also appears (see image below).
.
#arrowDown {
position: relative;
z-index: -100; /* z-index of rectangular div is 100 */
width: 88.5839416px;
height: 82px;
margin: -50px 100px;
bottom: 89.2px;
background: url('../img/arrowDown.png') no-repeat center center;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
-webkit-filter: drop-shadow(0 0 5px #FFFFFF);
-moz-filter: drop-shadow(0 0 5px #FFFFFF);
-o-filter: drop-shadow(0 0 5px #FFFFFF);
filter: drop-shadow(0 0 5px #FFFFFF);
}