the problem is that there is no distinction between a webapp and a document, the line is thoroughly blurred. For documents, you can probably just override a fair bit of schenanigans and things will still work, if look somewhat ugly, but for a spreadsheet app that's a rube goldberg machine of javascript and continuously regenerated html and css, it'll just turn it into a broken mess. the browser has become the de facto cross compatible application platform, which requires handing a lot of control to websites.