There has been more focus than ever these days not just on the look and feel (and usability) of web sites, but also on the underlying structure of the page - the standards compliant HTML that drives the site. Evolt has a new piece posted that can help to shed some light on why this subject has become so important.

Web technologies have always been misused, to achieve effects they were not (yet) designed to achieve. While a very common and natural drive in humans, and sometimes a very fruitful approach, creative use of computer tools has got important negative consequences on the quality of web products.

[...] All sorts of bad things happen when you do this, most of them related to the usefulness limits of non-intended use of technology: lessened impact on search engines, access difficulty for people with another browser, bandwidth waste, terrible difficulties in maintaining the content, changing the layout, and dealing with new access technologies.

Overall, the piece gives several very compelling reasons as to why you should make your site as "beautiful" on the inside as the outside. They also bring up a basic, very valid point of making things standards compliant - because it makes use of the features of HTML as they were designed, it is efficient in what it does, and it is future-proof.