my own personal (short) answer is that a “valid” html is safer, more portable and more “honest” to the final user. safer because sticking to the rules avoids trivial programming errors. more portable because all standard compliant browsers (not many today, but the number is increasing every day…) will render standard pages in the same way and testing will be easier. more honest because using browser-dependent “features” (here including the exploitation of browser-dependent “bugs”) is going to break the site as soon as the user tries to use another browser: the consequence is that the average user will revert to the previous, buggy one in order to view the non-standard compliant website, keeping it alive… and we all want non-standard compliant browsers to die, don’t we?

standards are there so that people do not need to reinvent the wheel or support someone else’s wheels, too…