It might be possible to use CSS to specify (or "suggest", if you prefer) position (and sizing) of web elements in such a way that the resulting page adapts well to different window sizes. But I've never seen it done.

Even the impressive "CSS Zen Garden" (which is better than most CSS you'll run into) does a lousy job of supporting window sizes. Most of them just refuse to resize at all. If you make your window even slightly narrower than the author intended, you just get a horizontal scroll bar, if you make it slightly bigger, you just get blank space on the side(s). They act more like a PDF, a big image, or an HTML table with widths all hard-coded as pixel counts than like web pages that are trying to make data available with hints at presentation (which is what I'd think is one of the supposed big points advocated by CSS purists).

Sites that use tables for layout have the distinct advantage that parts never end up overlapping each other (actually, you can make this happen, but everyone understands that the ways to do this are just "wrong" and so I never see it happen). Take this example from the Zen Garden. It actually supports wider window sizes (the only one I've found that does). Now slowly narrow the window that it is in. You'll probably see (perhaps depending on browser quirks) the "Design Pickup" menu start to slide behind the column next to it, while the text under the "So what is this about?" header gets narrower, leaving more and more blank space to its right. Eventually, the "Design Pickup" menu comes out the other side and you have the two side menues (that both refuse to get even slightly narrower) over the top of each other, both sets of text visible on top each other, leaving neither easy to read.

In my experience, when I find a site that has impressively bad layout, it almost always contains CSS. This is even more true when I use my Zaurus (PDA). Its screen is wide enough that most web pages are usable from it as-is... unless they use CSS for positioning or sizing. If a page contains CSS positioning, then on the Zaurus I usually end up seeing overlapping parts and text wrapped after every single word, even though there is blank space to the right of it on the screen.

CSS positioning just makes it way too easy to over-specify things or to create specifications that will make things overlap. The design of CSS affords (encourages) poor layout specifications. Perhaps when CSS and/or the general level of understanding of it become more mature, there will be well-understood practices to avoid overspecified/nonsense layouts and CSS positioning will be more likely to result in flexible pages.

With tables, most authors (of pages that contain information that I'm interested in) seem to understand that specifying a fixed width as a pixel count is a bad idea. The most common problem with tables is inclusion of elements that won't shrink (wide images or long lines of text lacking places to wrap). In my experience, these are less common and less trouble when present than CSS positioning.

I'd like a browser that, by default, will wrap long text elements even if it doesn't contain whitespace outside of PRE tags. Although this would mean that a big "ASCII art" diagram might be unreadable if my window is too narrow, my experience is that the vast majority of the time it would be an improvement. Mostly it would mean that text that (out of laziness or ignorance) was specified to not wrap enough would become quite readable. For the rare case of "ASCII art" too wide for viewing "full screen", the browser could provide a UI method to revert to old-style non-wrapping behavior so you could view the "ASCII art" in parts by scrolling sideways. Opera on the Zaurus actually supports this but only when you've requested that virtually all layout features are to be ignored (arranging the page as a single column with images shrunk to fit if required). I'd like this feature on my desktop browser and to have it be the default and available even when other layout isn't being ignored.