I continue to encounter web sites that appear not to function in, say, Firefox and Chrome, but work in Internet Explorer.
Well, here is my take. There is nothing, zero, zilch, nil that IE provides that Firefox, Safari and Chrome do not. Therefore, if you application only functions well in a particular browser but not others it is a demonstration to the world of your lack of knowledge and understanding of modern web technologies.
There is absolutely no excuse to require browser compatibility in this day and age.