From http://openclipart.org/css/default.css:

#search {
    position:               absolute;
    left:                   0;
    top:                    100px;
    z-index:                2

    width:                  100%;
    min-width:              700px;

    height:                 30px;
    min-height:             30px;

    background:             #94aed8;
    text-align:             center;
    border-top:             1px solid black;
    border-bottom:          1px solid black;

    padding:                0;
    margin:                 0;
}

The missing semicolon after "z-index 2" causes it and the following 'width' property to be ignored, 
which results in bad layout in Safari and Opera.

See http://bugzilla.opendarwin.org/show_bug.cgi?id=10555

P.S. The W3C Valid CSS badge at the bottom of the page is misleading.