Many websites offer cross browser testing by taking snapshots of a web page on various browsers and comparing them to find rendering issues. It works sort of, but when fixing a rendering issue its convenient to have access to a real browser.
On may mac, I have to setup multiple virtual machines to test IE browsers for compatibility issues, which sucks as a developer when you are running your web-server, IDE and a virtual machine.
Browser stack offers an interesting service where you can use different browsers from a browser. Its like actually testing on a browser with access to all the developer tools for a browser inside a browser. Wondering what would happen if I keep on opening browser stack sites inside a browser stack browser .