<div class="container">
  <h1 class="display-1">Display 1 <small>small text</small></h1>
<h1 class="display-2">Display 2 <small>small text</small></h1>
<h1 class="display-3">Display 3 <small>small text</small></h1>
<h1 class="display-4">Display 4 <small>small text</small></h1>
  
  
  <h1>This is a <code>h1</code> with <small>small text</small></h1>
  <h2>This is a <code>h2</code> with <small class="text-muted">small muted text</small></h2>
    <h3>This is a <code>h3</code> with <small class="text-muted d-block">small muted text on own line</small></h3>
  <h4>This is a <code>h4</code> with <small>small text</small></h4>
  <h5>This is a <code>h5</code> with <small>small text</small></h5>
  <p>This is a <code>p</code> with <small>small text</small></p>
  <p class="lead">This is a <code>p.lead</code> with <small>small text</small></p>
  <h1>This is a <code>h1</code> with <small class="mouse">small text with custom class <code>.mouse</code></small></h1>
  
  
</div>
<!-- /.container -->
.mouse {
/*  12px  */
  font-size:0.75rem;
}

External CSS

  1. https://jacoblett.github.io/bootstrap4-latest/bootstrap-4-latest.min.css

External JavaScript

  1. https://code.jquery.com/jquery-3.2.1.slim.min.js
  2. https://jacoblett.github.io/bootstrap4-latest/bootstrap-4-latest.min.js