With all your web apps and client side scripts, cross-browser testing is vital. Great were the roaring 90s when Internet Explorer was the only browser. But now the end user uses any browser, mostly either Chrome, Firefox, Internet Explorer, or Safari. Despite several standardizations, there is no guarantee that scripts in one browser would run in the other. And as an end-user, the message "this page works only in Firefox" is the biggest turn off for me.
Here is an easy way of testing how your web UI looks in different browsers without really installing the browsers in your computer. Through Browserling. It's an open source node.js based StackVM implementation by Peteris Krumins, and you can follow the growth of this project and all his node.js tools, here.