Check out example codes for "css div went over top menu". It will help you in understanding the concepts better.

Code Example 1

#navbar {
  background-color:#990000;
  position:fixed;  
  z-index:1; /*Add this*/
  width:100%;
  height:50px;
  text-align:center;
  vertical-align:middle;
  line-height:50px;
  top:0px;
}

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