You have two problems:
1. You fixed the ul element to the top instead the container (nav)
Change you CSS like this:
.nav
{
width: 100%;
position: fixed;
top: 0px;
left: 0px;
z-index: 9999;
}
ul
{
list-style: none;
text-align: right;
background-color: rgba(0, 0, 0, 0.6);
margin-top: 0px;
height: 50px;
}
2. You are creating grey for the navigation bar by defining half-opaque black... it means you will see thru that nav-bar evn thing blow it...
Change rgba(0, 0, 0, 0.6) to rgb(103, 103, 103)