Why does order of float div and non-float div matter only in some cases?

Question

I have a similar problem as CSS Auto Margin pushing down other elements: A right floating sidebar gets pushed down below the main non-floating content div. The answer propo

Answer 1

The reason this works is because your containing element has no height. When you have nothing but floated elements inside a containing element, it will collapse to 0 height. If you were, for example, to add overflow: hidden; to #fluidColumnContainer, it would act as a clear-fix, expanding the container to contain the floated elements. Then you would see the right-floated element drop down again.