For completeness, the CSS spec requires elements whose heights use percentage to ignore the percentage unless the parent element has a fixed height. This requirement is not enforced in HTML mode (quirks mode) but is enforced in XHTML mode (standards compliant mode). If you have been using percentages as a way to achieve full screen images, you should stop doing that for while it works in most readers, it is not actually standards compliant.
Use an SVG wrapper instead.
See
https://developer.mozilla.org/en-US/..._Mode_Behavior