If we want to produce machine readable texts:

• we have a reasonable notation, XML
• we have a character encoding system, Unicode
• we can use whatever elements we like in XML
• we know how to write XML processing applications

What more do we need? Well, we also

• want to interchange texts and tools with others
• need to have a formal way to express conditions about our markup
• should find a place to document our vocabulary

This is what a DTD or a schema provides: