contain scales the image, while preserving its aspect ratio, to the largest size such that the image fits within the background positioning area. cover scales the image, while preserving its aspect ratio, to the smallest size such that both its width and its height cover the background positioning area. When the value is one or two lengths or percentages, the first value gives the width of the image, and the second value, if present, gives the height. Any missing value is treated as ‘auto’. An ‘auto’ value is resolved using the aspect ratio of the image and its other dimension or, failing that, by treating it as 100%. If both values are ‘auto’, then the intrinsic size of the image should be used. If the image has an intrinsic size in only one dimension, then the other dimension is treated as ‘auto’. If the image has neither an intrinsic width or intrinsic height, then its size is determined as for ‘contain’.