Not everything a browser displays has to fit in the "page" paradigm.
In that respect, we've been working around the "page" paradigm since the web was practically born. It was a flawed analogy because, even back then, screen sizes and display tech varied among users. Designers still approach web design as if they are designing for print. I've always maintained that if the web were based on a vector technology (think PostScript, but obviously not PostScript) we would be in a much better place both design-wise and accessibility-wise. Content would flow in a much more controlled manner with much less room for browser interpretation and second-guessing. But people were still clinging on to the write once run anywhere (ahem, Java) naivety of the day. And likewise they really thought that you could divorce presentation from semantics and... have something that just worked? I guess? Just sprinkle on some afterthought CSS tech crap and no one will ever notice that the entire thing is flawed at a fundamental level.