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New custom attributes namespace restrictions
EPUB 3.3 disallows w3.org and idpf.org from custom attribute namespaces. See https://github.com/w3c/epub-specs/issues/1388
This is a change from 3.2 where only the following two specific URIs were forbidden:
- http://www.w3.org/1999/xhtml
- http://www.idpf.org/2007/ops