Web Design in a Nutshell, 3rd Edition

Percentage values

Percentage values follow the same basic positioning model as keywords, but they provide a more fine-tuned control over the image placement. Percentage values are given in horizontal/vertical pairs, with a default value of 0% 0%, which places the upper-left corner of the image in the upper-left corner of the element.

Each percentage value specified applies to both the background canvas area and the image itself. A few simple examples should make this clear.

• The percentage values 50% 50% place the center of the image in the center of the element.

• The percentage values 100% 100% place the bottom-right corner of the image in the bottom-right corner of the element.

• The percentage values 10% 25% match a point that is 10% from the left and 25% from the top edge of the image with the same point in the element.

As for keywords, when only one percentage value is provided, the other is assumed to be 50%.

It is fine to mix length and percentage values, which makes it easy to specify that an image should be centered horizontally in the element but appear exactly 25 pixels from its top edge. CSS 2.1 also allows length and keywords to be combined, but not all browsers support that combination as of this writing.