Semantic

The Semantic web, in essence, is the idea that information on the web should be delivered in a way that standardizes data so that people can more easily share data. This includes eliminating extraneous code, adhering to standards, and focusing on delivering content.

The idea of "Table-Appropriate" design comes from this movement. Back in the day, people used HTML tables to create complicated visual layouts. This adds a lot of code that a) is slow and messy, and b) uses HTML code to describe the visual display, rather than ONLY delivering the content. HTML markup should ONLY describe the hierarchy of the information.

So we only use tables to describe tabular data, clean up the markup just to describe the data, and then use CSS to transform that markup into something beautiful.

Extensible

Just think of this as future-proofing. By adhering to known standards, and separating form from function, your site can easily be updated, and will play nice with the rest of the web.

XHTML

Adhering to this strictest of web standards, ensures that the code is consistent, correct, and helps to set the foundation for compatibility with the widest array of platforms and browsers.

CSS

These are "Cascading Style Sheets", and are where all the visual magic happens. Because the site ONLY describes the data in a pure, plain, and coherent fashion, you could apply a new stylesheet and complete change the site, without ever touching the content.

Making sure that you are standards compliant is sooooo important. As browser makers slowly but surely move towards adopting W3C standards, sites that adhere to these standards will be much more compatible with all platforms and browsers. It will also push the Web as a whole forward, since more attention can be paid to sharing data and innovating new standards driven technologies.