f7723559_CSS_HTML_XHTML_Reference_Book__Description_and_notes
[Property/Values] background-position either one or two percentages or lengths (or one percentage and one length) or one of top, center, or bottom, and/or one of left, center, or right. Or use inherit. [Description and notes] 
for setting the physical position of a specified background image initial value: 0% 0%, if a single percentage is set, it is used for the horizontal position and the initial value of the vertical is set to 50%, if only one keyword is used, the initial value of the other is center; applies to block-level and replaced elements: not inherited; percentages refer to the size of the box itself
CSS HTML XHTML Reference Book
Description and notes