HTML Correctness

Generally, I like to follow rules. I also like to run a profitable business. Does it make business sense to spend time on HTML Correctness?

My homepage fails some WC3 HTML tests. Lots of them.

My use of CSS was better.

Long ago (June & July of 2006), I researched the question of whether SEO Errors Matter and how HTML Errors & SEO related. At the time, I concluded that they do not.

Any research out there to prove otherwise? I hope so since like I said, I'm looking for a business justification to clean up my HTML. I like to follow rules.