Variable/fixed image height

In the Kindle OS, images with a fixed height (and no specified width) that would exceed the screen width are resized proportionally to fit the screen width. In other words the fixed height is functionally overwritten to fit the image proportionally to the screen if honoring it (proportionally) would result in the image's exceeding the screen width. Otherwise, the fixed height value is honored as long as the implied image width is less than the width of the screen.

How would someone code this kind of conditional behavior in css for epub2 to acheive similar outcomes on non-Kindle, epub-based readers?