Compared to previous versions, Joomla! 1.6 templates now output cleaner code. Up to version 1.5, by default, Joomla! used a notoriously outdated method to display content, deploying HTML tables not just (as intended) to display tabular data, but for page layout purposes. It was only possible to get Joomla! to output clean code using so-called template overrides. These days, table-less layout is considered a much better, more flexible way to design websites. Luckily, Joomla! 1.6 is fully up to standards in this respect.
Web standards are specifications created by the World Wide Web Consortium (W3C), led by Tim Berners-Lee, the inventor of the web. Sites using clean, semantically correct code without browser-specific hacks load faster and can be accessed more easily through different devices and browsers, including screen readers. To find out more about web standards, have a look at http://www.webstandards.org/learn/faq. You can also find a good introduction...