Okay, I'm confused. Does ≮ open a tag? Does it matter if it's composed or
decomposed?
It does not open a XML tag.
It does matter if it's composed (won't open a tag) or decomposed (will open a tag, but with a combining character, invalid as an identifier start)
Conclusion1: blind normalizations of XML documents, as if they were plain-text documents, can break the XML well-formedness of these documents.... This is caused by the fact that plain-text documents can be parsed by units of grapheme clusters or combining sequences. But XML parsing stops at the one-codepoint character level, and ignores canonical equivalences.
Conclusion2: XML documents are not plain-text documents.
