On the date this article was published, this page was, according to the W3C Validation Service, using Valid XHTML 1.0 Strict markup. Will it remain that way...who knows? Read on for more about this subject...
At first glance, most would probably look at this page and perhaps think: "What's the big deal?" For a layout as simple as that, XHTML 1.0 Strict conformance should be a no-brainer." Well, not really. First off, there are some things over which you have little or no control.
For example, this site uses Drupal which in turn uses PHP to generate the XHTML statements. If Drupal did not generate XHTML strict conformant code, you had two choices - live with the non-conformant statements or modify the Drupal core to have it generate conformant statements. Fortunately, the Drupal core does generate valid XHTML 1.0.
Drupal, along with the majority of open source CMS (e.g., Joomla!, Wordpress,the Nukes, etc.) , support 3rd party plugins, or extensions (Drupal calls them modules), that enhance the core functions and/or provide features and functions not present in the core distribution. So, this is one more area where XHTML strict conformance can go awry. Fortunately, most of the 3rd party developers serving these communities produce conformant XHTML. In those cases where they do not, you again have the two choices - live with it or modify the code to generate valid XHTML.
Next is what I call the presentation layer. Most CMS's have theming or templating systems that control the layout and format of the data presented to the Web browser. During the development of themes and templates, care must be taken to adhere to the XHTML 1.0 rules. More and more theme/template designers are paying attention to XHTML conformance; however, these designers are still in the minority.
And finally, there is the actual generation of the CMS content. This is where one is most likely to introduce invalid XHTML 1.0. Most open source CMS have GUI editors that can be used to input and format content items. But, many of these GUI editors do not generate valid XHTML 1.0 in many cases. so, there is a high probability that content submitted via these GUI editors will introduce XHTML that will fail validation for one reason or another.
So there you have it. Even web pages with a layout and formatting as simple as this one requires a lot of attention to have it conform to XHTML 1.0 strict...and stay that way!