Floating elements within a div, floats outside of div. Why?

Question

Say you have a div, give it a definite width and put elements in it, in my case an img and another div.

The idea

Answer 1

W3Schools recommendation:

put overflow: auto on parent element and it will "color" whole background including elements margins. Also floating elements will stay inside of border.

http://www.w3schools.com/css/tryit.asp?filename=trycss_layout_clearfix