You define the syntax and structure of elements using a document type definition (DTD), and you declare that definition in a document using a document type declaration. We've seen that you use the <!DOCTYPE> element to create a document type declaration. This element can take many different forms, as you see here (here, URL is the URL of a DTD, and rootname is the name of the root element); we'll see all these forms in this chapter:
<!DOCTYPE rootname [DTD]>
<!DOCTYPE rootname SYSTEM URL>
<!DOCTYPE rootname SYSTEM URL [DTD]>
<!DOCTYPE rootname PUBLIC identifier URL>
<!DOCTYPE rootname PUBLIC identifier URL [DTD]>
To use a DTD, you need a document type declaration, which means that you need a <!DOCTYPE> element. ...