[Excavator]

Hello jeddi,
Do not use deprecated <center> tags!

To center an element you need three things:

1. a valid DocType
2. an element with a width
3. that elements right/left margins set to auto

Since your #banner gets it's 900px width from inline styling, adding this bit in red to your CSS will center it.

#banner {
    border: 3px solid black;
    display: none;
    /*top: 0;
    left: 0; */
    margin: auto;
    overflow: hidden;
    position: relative;
}