I want a two-column div layout, where each one can have variable width e.g.
The solution to this is actually very easy, but not at all obvious. You have to trigger something called a "block formatting context" (BFC), which interacts with floats in a specific way.
Just take that second div, remove the float, and give it
overflow:hidden instead. Any overflow value other than visible makes the block it's set on become a BFC. BFCs don't allow descendant floats to escape them, nor do they allow sibling/ancestor floats to intrude into them. The net effect here is that the floated div will do it's thing, then the second div will be an ordinary block, taking up all available width except that occupied by the float.
This should work across all current browsers, though you may have to trigger hasLayout in IE6 and 7. I can't recall.