I learned something the other day when we were fleshing out the background properties in the Almanac. It's such an intuitive thing, that I guess I just never stopped to think about it before.
By default, background-position is
0 0, the
top left corner. Now imagine you set it to
50% 0. That background image is now center top. It figures out the size of that image, and positions the center of it at the center of the element.
That's different than if you, say, had an
<img> and positioned it at
left: 50%; in that scenario, the left edge of the image would be at the halfway point. If you want to center it, you'll need to pull it back to the left (negative translate or negative margin).
Here's a visualization of that difference:
So if you set something like
background-position: 25% 25%, that positions the image such that 25% of the way across itself aligns with 25% of the way across the element. And the same vertically.
Nothing revolutionary here. I just think it's cleverly designed. You don't have to think about it much, because it just works like you would assume it works. Of course if I set
right top for the background-position the image is drawn in the right top corner. I don't think about how the browser must be calculating the image size and the element size and aligning axises at those positions.