This article explains what the issues are, and why you should and how you can, bring your pages into compliance with the HTML standards. The fundamental reason for HTML standards compliance is to ensure that you use only those elements and structures that are likely to be understood by the widest range of user agents. Similarly, if you use proprietary dialects of HTML, some user agents won't understand everything you write. There is however, a standard for HTML which the browser makers say they support, and validation points out the divergences in your HTML source. By adhering to the standards you maximise the accessibility of your work to the widest range of user agents, and therefore, users.
Some people take the view that checking their pages in the two major browsers is validation enough.
There are still dialect variants even within a single make of browser, across the different versions and platforms - let alone between the two. Choosing to adhere to any proprietary format, or none in particular, is very likely to cause a site to look worse under older browsers, or future browsers, than if the standards were followed. Achieving interoperability lowers costs to content providers since they must develop only one version of a document. If the effort is not made, there is much greater risk that the Web will devolve into a proprietary world of incompatible formats, ultimately reducing the Web's commercial potential for all participants. Each version of HTML has attempted to reflect greater consensus among industry players so that the investment made by content providers will not be wasted and that their documents will not become unreadable in a short period of time.