Quote Originally Posted by Thantos View Post
How things are to be laid out is actually very poorly defined and a lot is left up to the implementation.
CSS is actually quite rigid. But the most basic properties of the web, namely the diversity of devices (and thus sizes) make it impossible to go beyond a certain level of specification.

I'm not sure how much screen readers read the screen and how much they read the underlying HTML.
Screen readers read the screen. Aural browsers read the HTML - but AFAIK there's not a single non-experimental aural browser out there.