Impact of border property on top margin

Answer 1

Problem is your ul tag is not clear

body{
  height: 500px;
  width: 80%;
  margin-top: 0;
  margin-bottom: 0;  
  margin-left: auto;
  margin-right: auto;
  padding: 0;
  background-color: lightgray; 
}

.header {
  width: 80%;
  height: 100px;
  margin-left: auto;
  margin-right: auto; 

  background-color: yellow; 
  /*border: solid 1px black;*/  
}
ul{float: left;}

 

     
      
    
  
  
    

      

    
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