It's not unheard of. Many never ran into http://www.w3c.org to see if their site met the standards.
Bob
I created a ASP.NET web application that used some components (Infragistic, ComponentOne and WebGrid). I developed on Windows system with IE, now my customers are using client browser on Linux with Mozilla, firefox, .... It's bad, many things work not correctly. There are many bugs! It has so many bugs that i think i can not fix all. I wonder that if there is any library (add-in) that support to Mozilla, firefox in order that they work good as IE?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic