Description
Authors must produce their image files so that such metadata occurs before the image data in the image file. (Note: This is the default for most images already.)
User agents should ignore any layout-impacting metadata (such as orientation or resolution) that occurs after the image data begins in the file. (Note: This rule does not impact metadata that does not affect layout, such as color space information.)
This assumes that layout changes after all the image data has loaded are a problem, while color space changes after all the image data has loaded are a not problem. They are.
As an example, PNG requires that all color space-affecting metadata cHRM, gAMA, iCCP, cICP occurs before the image data (and before the palette, if there is one). This is done specifically to avoid a jarring repaint of the image in different colors.
Other image formats might not impose such a constraint, and thus images will be found where that metadata is after the image data.
Consider a 100-frame animation, which is also on a slow connection; once all the image data loads, suddenly the current frame, all subsequent frames (and for looping animations, all subsequent iterations) have a different appearance.
Suggested fix: delete the informative (and thus, non testable) note "This rule does not impact metadata that does not affect layout, such as color space information".