Basically, nothing's changed. All we did was take the single line and break it into pieces, and then stack those pieces on top of each other. Piece of cake.
Since the structure of the XML document can be specified in DTDs they provide a simple way to make it easier to exchange XML documents that conform to a DTD. For example, if two software systems need to exchange information, then if both of the systems conform to one DTD, the two systems can process information from each other. DTDs are not as powerful as some kind of schema architecture for XML, they don't support typing, subclassing, or instantiation mechanisms that a schema architecture must have.
DTDs are a simple way to make sure that 2 or more XML documents are of the same "type". Its a very limited approach to making "typed" XML documents shareable across systems. In the future some kind of schema system will be proposed by the W3C that should allow typing, instantiation and inheritance of information (in XML).
All of the advantages of XML outlined so far all make interoperability possible. This is one of the most important requirements for XML, to enable disparate systems to be able to share information easily.
is an indirect effect, of course, in that the margins (and so on) of an element can induce an offset for child elements. Vertical margins are still collapsed, as shown in Figure 8-18:
DIV {margin: 20px; padding: 20px;}
P {margin: 10px; padding: 10px;}
BODY {color: black;}
A:link {color: #808080;} /* medium gray */
A.external:link {color: #666666;}
A:active {color: silver;}
A:visited {color: #333333;} /* a very dark gray */
that the default value of none will be used. As
we've seen, that will turn the border off altogether.