All About XHTML 1.1 Modules

The XHTML 1.1 DTD is a driver DTD that includes DTD modules. A DTD module is a section of a DTD that has two parts—an abstract part, which specifies what the DTD does in human-readable language, and the module itself, which is a DTD fragment that contains element types, a set of attribute list declarations, and a set of content model declarations.

You can find the XHTML 1.1 driver DTD at http://www.w3.org/TR/xhtml11/xhtml11_dtd.html#a_xhtml11_driver, and the actual implementation of the XHTML 1.1 modules (there's one for forms, one for text, one for images, and so on) at http://www.w3.org/TR/xhtmlmodularization/dtd_module_defs.html#a_xhtml11_modules. You'll find the XHTML 1.1 modules listed in Table 17.1.

Each module has two parts, and I'll take a look at them here in overview.

XHTML 1.1 Abstract Modules

Each XHTML 1.1 module has an description called an abstract module that specifies what elements and attributes are in the module, and gives a minimal content model for each element. You can find these abstract versions of the XHTML 1.1 modules at http://www.w3.org/TR/xhtml-modularization/xhtml_modules.html#s_xhtmlmodules. An abstract module is really just a table listing the elements, attributes, and minimal content models in the module. For example, the Basic Text abstract module appears in Table 17.2.

The Basic Text abstract module in Table 17.2 lists two attribute groups—Common and Core. Here's what they contain:

• The Common attribute groups contains the class, id, title, dir, xml:lang, style, onclick, ondblclick, onmousedown, onmouseup, onmouseover, onmousemove, onmouseout, onkeypress, onkeydown, and onkeyup attributes.

• The Core attribute group contains the class, id, and title attributes.

The content models in Table 17.2 also use the Heading, Block, and Inline content models; here's what elements they contain:

• The Heading content model contains <h1>, <h2>, <h3>, <h4>, <h5>, and <h6>.

• The Block content model contains <address>, <blockquote>, <div>, <p>, and <pre>.

• The Inline content model contains <abbr>, <acronym>, <br>, <cite>, <code>, <dfn>, <em>, <kbd>, <q>, <samp>, <span>, <strong>, and <var>.

An abstract module like the Basic Text module is only so that you can find out what's in a module—it isn't actually used by any software, nor does it appear in any actual module implementation. What actually makes these modules work is their implementations.

XHTML 1.1 Module Implementations

As far as an XML processor is concerned, a module is really just a DTD fragment that is included in a driver DTD. This DTD fragment can contain element declarations, attribute declarations, and content model declarations. You can find the XHTML 1.1 module implementations at http://www.w3.org/TR/xhtml-modularization/dtd_module_defs.html. For example, here's the XHTML 1.1 images module, xhtml11-image-1.mod:

<!--
…………………………………………………………….
-->
<!-- XHTML 1.1 Images Module
……………………………………… -->
<!-- file: xhtml11-image-1.mod

     This is XHTML XHTML 1.1, a modular variant of XHTML 1.0.
     Copyright 1998-2000 W3C (MIT, INRIA, Keio), All Rights Reserved.
     Rovision: $Id: dtd_module_defs.html,v 1.2 2000/01/05 20:58:33
shane Exp $ SMI

     This DTD module is identified by the PUBLIC and SYSTEM
identifiers:
     PUBLIC "-//W3C//ELEMENTS XHTML 1.1 Images 1.0//EN"
     SYSTEM "xhtml11-image-1.mod"

     Revisions:
     (none)
     ……………………………………………………………
-->

<!-- Images

        img

     This module provides markup to support basic image embedding.
-->

<!-- To avoid problems with text-only UAs as well as to make
     image content understandable and navigable to users of
     non-visual UAs, you need to provide a description with
     the 'alt' attribute, and avoid server-side image maps.

-->

<!ENTITY % Img.element  "INCLUDE" >
<![%Img.element;[
<!ENTITY % Img.content  "EMPTY" >
<!ELEMENT img  %Img.content; >
<!-- end of Img.element -->]]&gt;

<!ENTITY % Img.attlist  "INCLUDE" >
<![%Img.attlist;[
<!ATTLIST img
      %Common.attrib;
      src          %URI.datatype;           #REQUIRED
      alt          %Text.datatype;          #REQUIRED
      longdesc     %URI.datatype;           #IMPLIED
      height       %Length.datatype;        #IMPLIED
      width        %Length.datatype;        #IMPLIED
>
<!-- end of Img.attlist -->]]&gt;

<!-- end of xhtml11-image-1.mod -->

Here's how that module is included in the XHTML 1.1 driver DTD:

<!-- Image Module ……………………………………….. -->
<!ENTITY % xhtml-image.module "INCLUDE" >
<![%xhtml-image.module;[
<!ENTITY % xhtml-image.mod
PUBLIC "-//W3C//ELEMENTS XHTML 1.1 Images 1.0//EN" "xhtml11-image-1.mod" >
%xhtml-image.mod;]]&gt;

Note the parameter entity xhtml-image.module here. This entity is set to "INCLUDE" by default, as you see here, but you can set it to "IGNORE" if you want to exclude the image module in a customized version of XHTML 1.1. That W3C calls the XHTML 1.1 DTD fully parameterized, and that's what it means—you can include or exclude modules just by changing parameter entities.

You'll find various suffixes used in the XHTML 1.1 driver DTD; for example, the name of the actual image module file is xhtml-image.mod. Here are the suffixes, such as .mod, that you'll find in the XHTML 1.1 driver DTD, along with what they mean. Using these suffixes, you can decipher what the various parameter entities in the DTD do:

Extending XHTML 1.1

As mentioned, extending XHTML 1.1 works much like extending XHTML 1.0, except that integrating the new element into the element hierarchy is a little different. For example, instead of modifying the declaration of the <html> element directly, you modify the Html.content parameter entity, which looks like this in XHTML 1.1:

<!ENTITY % Html.content "( head, body )" >

This means that I can modify the DTD that I developed earlier, extend.dtd, to work with XHTML 1.1 by modifying this content model, adding <doc:foot> and including the entire XHTML 1.1 DTD this way:

<!ELEMENT doc:foot (#PCDATA) >
<!ATTLIST doc:foot
    footattribute    CDATA   #IMPLIED
>
<!ENTITY % Html.content "( head, body, doc:foot )" >

<!ENTITY % xhtml11.dtd PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> %xhtml11.dtd;

Now I can use this new DTD as we've used the XHTML 1.0 version of extend.dtd:

<!DOCTYPE html PUBLIC "-//Extender//DTD XHTML-Extensions
1.0//EN" "http://www.starpowder.com/steve/extend.dtd" >
<html xmlns="http://www.w3.org/1999/xhtml"
xmlns:doc="http://www.starpowder.com/steve/extend.dtd">
    <head>
        <title>
            Extending XHTML
        </title>

        <link rel="stylesheet" href="extend.css" />
    </head>

    <body>
        <p>
            Here is some text.
        </p>
    </body>

    <doc:foot>
            This is the page's foot.
    </doc:foot>
</html>

And that's it for our look at XHTML. These two chapters have provided an introduction to the subject, and we've hit the major differences between HTML and XHTML. You're now ready to create XHTML documents of substantial complexity. However, there are more details in XHTML than we can cover in two chapters—for example, what attributes are required in each of the more than 100 XHTML elements. For more information, see the W3C XHTML sites using the various URIs that I've listed—everything you need is there.

In the next chapter, I'm going to take a look at two more popular XML applications: Resource Description Framework (RDF) and Channel Definition Format (CDF).