Bootstrap 4 Code

Output

<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.3/css/bootstrap.min.css" integrity="sha384-Zug+QiDoJOrZ5t4lssLdxGhVrurbmBWopoEl+M6BdEfwnCJZtKxi1KgxUyJq13dy" crossorigin="anonymous">

</head>
<body>
<div class="container">
<h3>Demo of text-md-center class</h3>
<p class="text-md-center">Aligned center in medium/large viewports and left in small viewports</p>
<h1 class="text-md-center">h1: centered in medium/large screen and left in smaller</h1>
<h4 class="text-md-center">h4: centered in medium/large screen and centered in smaller</h4>
<h5 class="text-md-center">h5: centered in medium/large screen and centered in smaller</h5>
<h6 class="text-md-center">h6: centered in medium/large screen and centered in smaller</h6>
</div>
</body>
</html>

Demo of text-md-center class

Aligned center in medium/large viewports and left in small viewports

h1: centered in medium/large screen and left in smaller