Precisely 11 years ago, during a discussion with my manager at Grif SA (Jean Paoli, now working at MSFT, editor of XML 1.0 spec), we listed a few known big lacks of SGML, in particular in wysiwyg editing environments. When I say "known", it means that all the people working seriously on SGML knew about those problems. Eleven years. Today, sketching an implementation design based on XML Schemas, I have hit one of them. Strictly unresolved, it's almost a blocker for wysiwyg XML editing based on XML Schemas + CSS without prior knowledge of the schemas... You win a mention in this blog if you find the problem I hit. Make your guess at daniel AT glazman DOT org.

Update (Aug 16, 11:16am GMT+1): 21 proposals (all proposals got an email back) but no solution yet :-)