I have just been playing around in FF32 & Safari 7, with position: sticky. And it works perfecly with top: 0px or left: 0px (to st
position: sticky
top: 0px
left: 0px
If you know the height of the sticky element then you can use something like this:
.sticky-element { position: sticky; top: calc(100vh - 50px); }
This would also work with Stickyfill. Not sure about browser support though.