See if changing the height of the li makes any difference like this, also add a border to any CSS this will tell you which element is incorrect
li {
margin: 0;
padding: 0 0 6px 0;
position: relative;
list-style: none;
list-style-image: none; height:30px; border:1px solid black;
}
If it's not the height then try other values like top, bottom margins and padding
Good page to use for testing is here
http://www.w3schools.com/html/tryit.asp?filename=tryhtml_iframe_height_width