Was just digging into this same problem myself. Seems like the problem with CSS is that it's all about text size, and not page size. There are a lot of scenarios I can think of with books where page size is more important - cover pages like this example, Children's books, other illustrated material, etc. Is SVG the only way to create content where everything scales relative to the page size?
I just tried out that svg cover. Looks great in Firefox, but Adobe DE mangles it in both the desktop app and my prs-505. Will SVG work with simpler layouts?