h1, p {
    background-color: #DDE0E3; /* Light grey */
}

h1 {
    padding: 50px;
    border-bottom: 1px solid #5D6063;
}

p {
    /*   
    
    padding-top: 20px;
    padding-bottom: 20px;
    padding-left: 10px;
    padding-right: 10px; 
    
    padding: 20px 10px;   Vertical  Horizontal 
    
    */
    padding: 20px 0 20px 10px;  /* Top  Right  Bottom  Left */
    margin: 50px 50px;
}

em, strong {
    background-color: #B2D6FF; /* Light blue */
    display: inline;
}

div {
  color: #FFF;
  background-color: #5995DA;
  font-weight: bold;
  padding: 20px;
  text-align: center;
  border: 2px solid #5D6063;
  border-radius: 5px;
  width: 200px;
  box-sizing: border-box;
  /* box-sizing: content-box; Default behaviour */
}

body {
    text-align: center;
}

h1, p {
    background-color: #DDE0E3; /* Light grey */
}

h1 {
    padding: 50px;
    border-bottom: 1px solid #5D6063;
}

p {
    /*   
    
    padding-top: 20px;
    padding-bottom: 20px;
    padding-left: 10px;
    padding-right: 10px; 
    
    padding: 20px 10px;   Vertical  Horizontal 
    
    */
    padding: 20px 0 20px 10px;  /* Top  Right  Bottom  Left */
    margin: 50px 50px;
}

em, strong {
    background-color: #B2D6FF; /* Light blue */
    display: inline;
}

div {
  color: #FFF;
  background-color: #5995DA;
  font-weight: bold;
  padding: 20px;
  text-align: center;
  border: 2px solid #5D6063;
  border-radius: 5px;
  width: 200px;
  box-sizing: border-box;
  /* box-sizing: content-box; Default behaviour */
  margin: 20px auto; /* Vertical  Horizontal */
}

Add horizontal aligning using auto margin
Add horizontal aligning using auto margin
