noted in NSN4

Unless margin-left is explicitly set to a non-zero value on Hn
(heading) elements, vertical margins can't be set properly.

Contrary to the CSS1 spec, em and percent measures do not refer to the
font size of the element itself when the font size of the element is
specified in em or percent. Em and percent measures apparently refer to
the closest ancestor with an absolute font-size declaration.

David Perrell