This often happens if you are building a site. Non-compatibility to multiple browsers normally does not effect the functionalities of a website. Rather it mostly affects the design view of a site, which creates the first long lasting impression on a viewers mind. A website may be displayed perfectly in a particular version of IE, but may look much different in Fire fox and even in other versions of IE also. The whole thing depends on the level of complexity of the design. In case of simple designs, if some conventions like W3C validation and standard resolution of 1024 pixels X 768 pixels are followed during the development process, normally it does not create any problem. But in case of a complex design, lot of other things should be taken care of. Now, to the best of my ability I shall try to write down some suggestions for the site owners and the web designers. It is nothing new; but may act as a reminder during the development process.
My suggestions for the website owners: It is a common practice for a good web development company to make all the websites compatible to two major browsers, named Internet Explorer and Mozilla Firefox. Still I like to say a few words to the respectable site owners.