HTML errors will cause different problems on different browsers and with different content. The browser always tryes to figure it out, but depending on the context will reach different conclusions.
In your first linked page, you have an extra </div> at line 372 which is at the bottom of the content area. It destroys the nesting of the containers.
You also have an opened <i> tag in a widget ( <i>Thema Unerwünschte Bildelemente…) which is never closed