Logical structures of documents are described by existing markup languages, e.g., SGML(Standard Generalized Markup Language), XML(Extensible Markup Language), where element structures are specified by DTD(Document Type Definition) and actual document instances are marked up by tagging with the element types defined in the DTD.
HTML(HyperText Markup Language) is an instance of SGML with a fixed DTD. Therefore, HTML can be used to describe logical structures of a simple document.
Examples: