If we use a gradient as a background image, and set one of the background size dimension to auto, we currently scale that dimension by the same amount as the other axis to preserve the aspect ratio. Gradients have no natural dimensions (and thus no natural aspect ratio) - https://w3c.github.io/csswg-drafts/css-images/#gradients. Background sizing should fall back to using 100% (for the case where we have one size, and auto), if there's no natural aspect ratio or dimensions - https://w3c.github.io/csswg-drafts/css-backgrounds/#background-size <rdar://101661731>