The Web works with valid and invalid HTML. So why is valid HTML important? And how does invalid HTML affect everyone who uses the Web? Technical standards are the bedrock of innovation and it is a recognized fact that smaller companies often lead in the creation of innovative technologies. Standards create a level playing field on which these smaller companies compete with giants.
Non-standard use of HTML on the Web thus makes it extremely difficult for smaller companies to compete with their larger competitors, because an enormous amount of their development resources must go to dealing with invalid HTML. The result is that we have less competition, a limited choice of applications, and fewer innovative technologies.