@arspr: A nesting error is only an error at the point where a mismatched closing tag is present, not where the opening tag is present. Therefore the behavior of Check Book is correct.
As for xmlns:xml, that declaration is redundant and should be removed. The xml namespace is *always* defined in XML documents.
http://www.w3.org/XML/1998/namespace
I suppose I could have the html 5 parser remove it automatically, I will look into that.