Can you create gradients that fade to opacity using CSS or JavaScript?

Yes

for the colors, use rgba(x, y, z, o) where o is the opacity

should work

e.g.

background-image: -webkit-gradient(linear, left top, left bottom, from(rgba(255, 255, 255, 1)), to(rgba(0, 0, 0, 0)));

Edit:
For the final value (opacity) 1 is opaque & 0 is transparent