For any site - be it Drupal or non-Drupal, this is most probably due to simple mistakes made while entering attributes. Let us see one very common case where a simple mistake can cause glaring issues:
We usually see the css given something like this. We do not see any glaring issue in the class defined below.
#header {
background:url("images/headback.png")repeat-x;
width:1020px;
height:120px;
font-family:arial;
position:relative;
}
