background-position doesn't support "center 50% top". This is also the case for basic shapes that use the same syntax.
Ditto for center "50px center 50%".
(In reply to comment #1) > Ditto for center "50px center 50%". Sorry, meant "center 50px center 50%".
"top 50px", "center 50% top 50%" don't work either
Offsets from center are not allowed as part of the background position syntax. See: http://www.w3.org/TR/css3-background/#the-background-position
(In reply to comment #4) > Offsets from center are not allowed as part of the background position syntax. See: > http://www.w3.org/TR/css3-background/#the-background-position That's the point. Closing the bug as invalid.