Web-based software vendors have a pressing concern that other businesses do not have – the cross-browser issue. A solution that end-users access via their web browser of choice must function in all possible browsers, which can pose a real challenge to the provider. According to TechNewsWorld they 'must contend with hard-to-plug holes that can open up in cross-platform programs such as Web browsers.' This is such a common problem that companies have developed browser testing tools and services to respond to the need in the market.
There is a balance you have to live with between being cool and pretty and functional (e.g. using AJAX so you have a slick interface like gmail) and working on any ancient browser someone dredges up.
Sometimes it's hard. Our code must work on all browsers, in all countries, everywhere. This is hard.
Smartphones are making it much harder. But that's another story.