There is no MPEG-4 iTunes metadata standard. Mutagen’s features are known to lead to problems in other implementations. For example, FAAD will crash when reading a file with multiple “tmpo” atoms. iTunes itself is our main compatibility target.
Compatibility / Bugs
Prior to 1.16, the MP4 cover atom used a .format attribute to indicate the image format (JPEG/PNG). Python 2.6 added a str.format method which conflicts with this. 1.17 provides .imageformat when running on any version, and still provides .format when running on a version before 2.6.