Nothing really, but a lot of users may be stuck on older browsers. With government agencies or banks or the like, they tend to not upgrade until the last day that their current setup is supported. The cost of upgrading infrastructure is far too great to be justified by non-tech savvy higher ups who fail to understand security risks of not regularly upgrading systems.<p>So, developers are stuck programming for some ancient godawful version of Internet Explorer that barely even supports ES5.