This might not come as a surprise to anyone but when I found this out I thought it was cool.
If you apply a
filter: drop-shadow(h-shadow v-shadow blur color);style to a .PNG image with an alpha channel then it will render the drop-shadow on the actual image rather than a box around its border. You can see this in the following example:
filter: drop-shadow(0px 0px 50px yellow);
Pretty neat but probably useless.