Controlling the browser gives Microsoft an edge when it comes to setting standards. They would like nothing better than to have everyone creating web pages with their software that can only be loaded with their software. Once they have the content encoded in their proprietary formats (such as they do with Word and Excel), every couple of years they turn over a ton of $$ by forcing everyone to upgrade. Those Office file formats are legendary for being so badly documented that even Microsoft has trouble writing filters for it, and I have yet to see a 3rd-party filter that is 100% faithful in rendering Office documents (especially Word).