Check out example codes for "how to center div". It will help you in understanding the concepts better.

Code Example 1

//HTML
<div class="parent">
	<span>Hello World</span>
</div>

//CSS
.parent {
  display: flex;
  justify-content: center;
  align-items: center;
}

Code Example 2

<!--center text-->
<!--padding deals with margins within the element itself-->
<!--margin deals with blank space surrounding and element-->
<!DOCTYPE html>
<html>
<head>
<style>
.center {
  margin: auto;
  width: 80%;
  border: 3px solid #73AD21;
  padding: 10px; 
  margin: 20px;
}
</style>
</head>
<body>

<h2>Center Align Elements</h2>
<p>To horizontally center a block element (like div), use margin: auto;</p>

<div class="center">
  <p><b>Note: </b>Using margin:auto will not work in IE8, unless a !DOCTYPE is declared.</p>
</div>

</body>
</html>

Code Example 3

# center text
.center {
  text-align: center;
  
border: 3px solid green;
}

Code Example 4

<!DOCTYPE html>
<html>
<head>
<style>
.center {
  margin: auto;
  width: 80%;
  border: 3px solid #73AD21;
  padding: 10px; # deals with margins within the element itself
  margin: 20px; # deals with area around outside of element
}
</style>
</head>
<body>

<h2>Center Align Elements</h2>
<p>To horizontally center a block element (like div), use margin: auto;</p>

<div class="center">
  <p><b>Note: </b>Using margin:auto will not work in IE8, unless a !DOCTYPE is declared.</p>
</div>

</body>
</html>

Code Example 5

<!DOCTYPE html>
<!--clearfix-->
<html>
<head>
<style>
div {
  border: 3px solid #4CAF50;
  padding: 5px;
}

.img1 {
  float: right;
}

.clearfix {
  overflow: auto;
}

.img2 {
  float: right;
}
</style>
</head>
<body>

<p>In this example, the image is taller than the element containing it, and it is floated, so it overflows outside of its container:</p>

<div>
  <img class="img1" src="pineapple.jpg" alt="Pineapple" width="170" height="170">
  Lorem ipsum dolor sit amet, consectetur adipiscing elit. Phasellus imperdiet, nulla et dictum interdum...
</div>

<p style="clear:right">Add a clearfix class with overflow: auto; to the containing element, to fix this problem:</p>

<div class="clearfix">
  <img class="img2" src="pineapple.jpg" alt="Pineapple" width="170" height="170">
  Lorem ipsum dolor sit amet, consectetur adipiscing elit. Phasellus imperdiet, nulla et dictum interdum...
</div>

</body>
</html>

Code Example 6

<!--center text-->
<!DOCTYPE html>
<html>
<head>
<style>
.center {
  text-align: center;
  border: 3px solid green;
}
</style>
</head>
<body>

<h2>Center Text</h2>

<div class="center">
  <p>This text is centered.</p>
</div>

</body>
</html>

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