Might be a bug, in that case you have to report it to the authors.
The problem with building a good checker is that the standards are ambiguous. Several items can be read/implemented in multiple manners, all correct according to the standards. Another issue is that a lot of readers (specifically iBooks) ignore standards to implement their own version of it.
The addition of Javascript is, in my opinion, a downgrade just as adding audio/video is. There were enough other strange/incomplete issues with the format to solve first.
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