That means you need valid code with no errors: the ePub standard requires XML error handling, so you can't get away with HTML 4.0» «style tag soup. (alistapart.com)
So, here is a new way to create valid code in your KF8 files that will degrade gracefully for older Mobi - only devices. (ebookarchitects.com)
This includes inflatable rear outboard safety belts, pre-collision assist with pedestrian detection, and the Enhanced Security Package which requires a valid coded key to avoid drive away theft and electronic lockable storage. (bestride.com)