The code might includes many mistakes, however what makes me crazy is the fact, that if I make the Browser window smaller, the sidebar (in pink) jumpt underneath the main content.
Hello kayut,
When #wrapper narrows up to where your #main and aside do not fit beside each other, the elements start re-arranging themselves so they do fit. That's what they're supposed to do.
To stop that, set either a width or a min-width on #wrapper.