Bootstrap class: .display-# (1-4)

<h1 class="display-1">Display 1</h1>
<h1 class="display-2">Display 2</h1>
<h1 class="display-3">Display 3</h1>
<h1 class="display-4">Display 4</h1>

Preview

You don't need to remember all CSS classes. Just use the Bootstrap Editor instead.