One problem with that idea is that tools like CSS tables still have some dependency on the source order in the HTML. So do certain other popular tools, notably

One problem with that idea is that tools like CSS tables still have some dependency on the source order in the HTML. So do certain other popular tools, notably various aspects of float positioning. If we're going to have a proper separation of content from presentation, such that we can do things like alternative layouts for responsive design or A/B testing using only the presentation layer, there has to be a viable mechanism for overriding that sort of source-order dependency.