Be the browser’s mentor, not its micromanager. - Build Excellent Websites
This one-page site that Andy has made to illustrate his talk at All Day Hey is exactly what I was talking about with declarative design.
Give the browser some solid rules and hints, then let it make the right decisions for the people that visit it, based on their device, connection quality and capabilities. This is how they will get a genuinely great user experience, rather than a fragmented, broken one.