background-size [ , ]*;]]>]]>[ <length> | <percentage> | auto ]{1,2} | cover | contain

The first value gives the width of the corresponding image, the second value its height. If only one value is given the second is assumed to be auto.

If both values are ‘auto’ then the intrinsic width and/or height of the image should be used, if any. If the image has neither an intrinsic width nor an intrinsic height, its size is determined as for contain.

]]>]]>]]>auto value for one dimension is resolved by using the image's intrinsic ratio and the size of the other dimension, or failing that, using the image's intrinsic size, or failing that, treating it as 100%.]]>