XML Reference Guide

Mar 14, 2003

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Because we already have a Reference Guide section on XHTML, you may be wondering why we have a separate one for XHTML 2.0. After all, wouldn't it make sense to just update the other section?

Well, no.

When it comes to actual usage, XHTML 2.0 is a whole new animal. Or more accurately, XHTML 2.0 is not the same old animal. And that's what's really different about it. Over the years one of the guiding principles of HTML is that it has always been backwards-compatible. The newest browsers always read the oldest pages correctly.

Well, OK, they would read the old pages correctly, if the old pages had been coded correctly. That's actually how we got from HTML 4.01 to XHTML 1.0. Most of the "flexibility" that came from HTML's SGML roots, such as case-insensitivity, optional closing tags, and optional quotes around attribute values, was removed when HTML was subjected to the rules of XML and become XHTML 1.0. Creating a valid XHTML 1.0 page means no more being lazy about overlapping tags, and other bad habits. Also, structure was emphasised over presentation, which meant that you should be writing:

<&h1>The next steps</h1>
<p>Once you've reversed the polarity of the 
neutron flow, open the rotor and restart the 
temporal oscillator.</p>

and not

<p style="font-weight: bold; font-size: 14pt;">The next steps</p>
<p>Once you've reversed the polarity of the 
neutron flow, open the rotor and restart the 
temporal oscillator.</p>

But still, XHTML 1.0 and HTML 4.01 were pretty similar. A browser that understood one could understand the other, as long as you weren't requiring well-formed XML.

In most ways there was even less change between XHTML 1.0 and XHTML 1.1. In fact, the specs are the same, as far as content is concerned. The only difference was the way in which XHTML was "modularized" in definitions, and that has zero effect on most web developers.

And then came XHTML 2.0. Still in working draft status, XHTML 2.0 has one major, major difference from all that has come before.

It's not backwards compatible.

And we're not just talking about obscure tags like <big> here. We're talking major changes like the elimination of <b>, <i> and even <img>. Frames are gone completely (and good riddance), replaced with their next-generation counterpart, XFrames. Ditto for forms. This is not your father's HTML, folks.

The first major change, as I said, is in the elimination of presentation-related tags such as the bold and italics tags. That means you're going to be getting more use out of the seemingly useless <span> tag. In other words, instead of writing:

<p>Structure is <b>good</b>.</p>

You need to be writing

<style type="text/css">
    .strong {font-weight: bold}
</style>
<p>Structure is <span class="strong">good</span>.</p>

(Or better yet,

<p>Structure is <strong>good</strong>.</p>

)

Images can now be added using the <object> tag, which enables you to present different levels of content based on the capacity of the browser, as in:

<object data="starfield.mov" type="application/mov">
    <object data="starfield.jpg" type="image/jpg">
        NASA has released new Hubble telescope pictures, strung
        together as an animation.
    </object>
</object>

This method enhances accessibility, since authors can automatically provide alternate content for users who may be using text-only or even voice-only browsers. The browser starts at the outer tag (in this case, the movie) and keeps looking inside until it finds content it can handle.

XHTML 2.0 is all about adding meaning to your documents. For example, where you used to have six levels of headings, you can now explicitly organize your content into sections, as in:

<section>
   <h>Overview</h>
   <section>
        <h>What Is XML?</h>
        <p>A cousin of Hypertext Markup Language 
        (HTML),...</p>
   </section>
</section>
<section>
   <h>Table of Contents</h>
   <section>
         <h>The Document Object Model</h>
         <p>The most common methodology for writing 
         XML programs is the Document Object Model (DOM). ...</p>
         <section>
               <h>InformIT Articles and Sample Chapters</h>
               <p>"XML in .NET: The DOM Interface," by Dale 
               Michalk, is a look at the Document...</p>
         </section>
         <section>
               <h>Books and e-Books</h>
               <p>The first two sections of ...</p>
         </section>
         <section>
               <h>Official Documentation</h>
               <p>The W3C maintains a Document Object Model 
               main page that...</p>
         </section>
   </section>
   <section>
         <h>DOM and Java</h>
...
   </section>         
</section>

The advantage here is that search engines and other automatic programs can understand how pieces of information relate to each other, and sections are self-contained.

There's also a major change when it comes to linking. Although the <a> tag has not been removed, or even deprecated, it's now possible to make something a link without even using it. For example, in HTML you need to make an image a link like this:

<a href="home.html">
    <img src="/images/logo.gif" alt="Logo" />
</a>

In some browsers, that carriage return after the image (but before the closing tag for the link) caused an annoying blue underline to appear on the page. In XHTML 2.0, you can just say:

<img src="/images/logo.gif" alt="Logo" href="home.html" />

In fact, in XHTML 2.0, anything can become a link this way, including entire sections or even the whole body of the page.

Other major changes? Well, as I said, frames have been removed. (They may eventually be replaced by XFrames, which are not officially part of XHTML 2.0 but provide solutions for problems such as bookmarking a page the user has already changed.) Forms have been replaced with XForms, which provide much more robust data validation and event handling without resorting to JavaScript. And speaking of events, one of the goals of XHTML 2.0 is to minimize reliance on scripting by incorporating XML Events. They've also taken great pains to increase usability and accessibility by emphasizing such information as captions and summaries.

XHTML 2.0 will be a while in coming, as no browsers currently support it, but it pays to understand the changes, so you can at least think about them when you're coding your current pages, and hopefully build them in such a way that any changes will be easy to implement.

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