Startup Idea For Helping People Design/Test Websites Faster

I'm designing a website and I need a product that I couldn't find. Essentially, I want to test whether a webpage I've designed looks identical (or close to it) in different browsers or not. Let's forget Javascript/AJAX for a moment, just how the page looks when it loads.

I need a website where I can enter my URL, choose 2 browsers (say IE7 and FF4) and see 3 horizontal tabs - how the page looks in IE7 (image file), how it looks in FF4 and the difference between those images (think of pixel-by-pixel difference i.e. empty when the images are identical at that pixel and some colored pixels when they're not).

While there are some sites that do some parts of it (e.g. http://browsershots.org/), no-one really does what I'm trying to do. I think there would be a market for a SaaS startup doing what I described above. What do you think?