I'm still utterly unconvinced "errors" are ever a good idea.
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So "error messages" being output at all (except maybe in "action=debug", and even then that should be more "lint"-like) is fundamentally erroneous.
That argument works for arguing against refusing a save invalid content, but I was explicitly not suggesting that. My suggestion was to let everything through, but display error messages (or, rather, warnings) so people can easily tell if something isn't displaying right.