From the CSS Specification: An "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%.
From the CSS Specification: An "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%.