Extending Public XHTML DTDs | Real World XML (2nd Edition)

In the previous section, I created a new DTD to extend XHTML, and I put it to work in an XHTML document like this:

 <?xml version="1.0"?>  <?xml-stylesheet type="text/css" href="ch17_19.css"?>  <!DOCTYPE html SYSTEM "ch17_17.dtd">  <html xmlns="ch17_17.dtd" xml:lang="en" lang="en">     <head>         <title>         </title>     </head>     <body>         <p>             This text uses a new XHTML element for             <underlinedredtext>emphasis</underlinedredtext>.         </p>     </body> </html>

In this case, I declared a local DTD for private use with the SYSTEM keyword. You can also make your DTD public with the PUBLIC keyword, but to do that, you've got to create a formal public identifier (FPI). Here are the rules for FPIs:

The first field in an FPI specifies the connection of the DTD to a formal standard. For DTDs that you're defining yourself, this field should be [ms] . If a nonstandards body has approved the DTD, use + . For formal standards, this field is a reference to the standard itself (such as ISO/IEC 13449:2000 ).
The second field must hold the name of the group or person that is going to maintain or be responsible for the DTD. In this case, you should use a name that is unique and that identifies your group easily (for example, the W3C simply uses W3C ).
The third field must indicate the type of document that is described, preferably followed by a unique identifier of some kind (such as Version 1.0 ). This part should include a version number that you'll update.
The fourth field specifies the language your DTD uses (for example, for English you use EN ). Note that two-letter language specifiers allow only a maximum of 24 x 24 = 576 possible languages. Expect to see three-letter language specifiers in the near future.
Fields in an FPI must be separated by a double slash ( // ).

You can put these rules to work, changing the previous example to indicate that the DTD we're using is publicly available, like this:

Listing ch17_21.xml

 <?xml version="1.0"?> <?xml-stylesheet type="text/css" href="ch17_19.css"?>  <!DOCTYPE html PUBLIC "-//starpowder//ELEMENTS XHTML-Custom 1.0//EN"  "http://www.starpowder.com/DTDs/ch17_17.dtd"> <html xmlns="http://www.starpowder.com/DTDs/ch17_17.dtd" xml:lang="en" lang="en">     <head>         <title>         </title>     </head>     <body>         <p>             This text uses a new XHTML element for             <underlinedredtext>emphasis</underlinedredtext>.         </p>     </body> </html>