Web standards aren’t just an abstract idea (though the creators of Internet Explorer seem to think that they are). There are a concrete set of rules for developing well formed markup in html which were established by the WC3. Most modern browsers follow this standard when determining how an html page will display to the user; however there are some browsers (IE in particular) which have snubbed their noses at the standard. This makes building standard layouts that work in all browsers particularly challenging, and it is the main reason that my job exist. I insure that my code is WC3 validated as XHTML 1.0 strict by validating them with the WC3 validation service.
In summary, there really isn’t much room for argument when it comes to web standards. All pages should validate as WC3 compliant to insure that your website succeeds.
(Note: there are many contributed modules for Drupal that output html. Most are standards compliant, however there are some exceptions. If a module outputs non-standard code within a themable function I will override it.)