Reason
The DTD tells the browsers about the mark-up language in the document.
- The browsers "see" the DTD immediately at the top of the document.
- The first part of the DTD tells them which markup language has been used.
- But what if a browser does not know the language?
- The second part of the DTD tells them where to find the language definition (like a dictionary), which a browser can fetch and apply instantaneously.
