CHAPTER  2 Well-Formed Documents and Namespaces WITH BASIC DEFINITIONS and examples behind us, we can move on to a detailed discussion of the specifications. In this chapter, we concentrate on documents without DTDs because they have a simpler structure. Although occasionally mentioned in this chapter, DTDs and other approaches to validation (such as XML Schema and RELAX NG) will be introduced in Chapter 3. In outline, this chapter proceeds as follows: •   HTML vs. XHTML •   XHTML modularization and XHTML Basic •   well-formed XML documents •   names and namespaces •   global attributes and XLink •   namespace URI and RDDL (XHTML Basic + XLink) We will start with a comparison of HTML and XHTML. HTML, XML, and XHTML HTML is by far the most familiar markup language. We will review its main fea- tures in comparison with XHTML to emphasize, one last time, the following basic facts. •   HTML is a specific language defined in the SGML framework. •   XML is not a language but a framework for defining languages. •   XML is a revision of SGML. 41