Because the customer's perception of your professionalism matters, and the problems with the browser's execution of javascript would strongly and negatively aff

Because the customer's perception of your professionalism matters, and the problems with the browser's execution of javascript would strongly and negatively affect their likelihood to use your service and convert to a paying customer? My site depends heavily on custom-coded javascript. Minor browser inconsistencies are one thing you expect to handle or live through. Absolute FAIL because of poor javascript engine implementation is another.