There does exist a subtle middle ground between proper syntax and proper semantics, namely well-formedness. Well-formedness is technically a part of syntax rules (i.e. syntactic correctness) but not a part of formal grammars and other similar stuffs, making it harder to classify. For example, XML's opening tag and closing tag should be matched like <foo></foo>, but this syntactic rule is not described in the formal context-free grammar. It is possible to make a formal context-sensitive grammar that only accepts a well-formed syntax, but that would make the specification unnecessarily complex, hence the introduction of informal rules. Some still may argue that it is actually kind of semantics, however.