There's a difference between a fixed width layout and an absolute positioned layout - the fixed width layout can adapt.
The biggest issue with absolute positioned layouts (as with design for print) is they can only scale, but not adapt to the various resolutions and pixel densities we encounter on clients viewing websites.
Web design is first most the design of a pattern of design in order to achieve consistency via reuse. If everything has fixed dimensions; it wouldn't be very reusable.
On the topic of using JavaScript to perform the layout:
1. JavaScript performance in newer browsers are pretty good, but ranging from extremely to pretty bad on older browsers (don't forget the mobiles!)
2. You'll have to resort to using CSS anyway for specific layouts (ex. keeping an element always visible) as repositioning an element on streaming events using JavaScript tends to be really slow.
3. The amount of reuse you'll get from even writing a decent interface for layout functions will be pretty low.
4. Behavior of CSS using a reset is more consistent than JavaScript implementations. You'll probably need to normalize between different browser versions - that means using something like jQuery, Moo Tools etc. which adds a hefty payload.
5. Waiting for your JavaScript to be downloaded and interpreted to reposition elements (after the DOM has already rendered) will be an ugly sight.
You should invest time in pre-processors such as Stylus, Less or SCSS (you can write reusable mixins that you can include on different projects and use variables to customize them). If you haven't, you should read up on object-oriented CSS: http://coding.smashingmagazine.com/2011/12/12/an-introductio...