Google Docs use normal divs on Chrome and iframe on Firefox for presentation.Can you explain why?

Question

This is especially important for text editing. Using iframes for text editing (lets say you will develop a rich text editor) looks like a common and reliable technique. Text editing in Google Docs presentation differs from Chrome to Firefox. They use iframes in Firefox which seems better but in Chrome they do this by putting manually defined span and font tags. Why do they do this since both browsers and also IE support text editing on iframes