Buttons | Bootstrap 4 (BFD) on Drupal 9

Normal Buttons

btn (basic) btn-primary btn-secondary btn-success btn-info

btn-warning btn-danger btn-light btn-link

<a class="btn btn-primary" href="#">Button Text</a>
<a class="btn btn-secondary" href="#">Button Text</a>
<a class="btn btn-success" href="#">Button Text</a>
<a class="btn btn-info" href="#">Button Text</a>
<a class="btn btn-warning" href="#">Button Text</a>
<a class="btn btn-danger" href="#">Button Text</a>
<a class="btn btn-light" href="#">Button Text</a>
<a class="btn btn-link" href="#">Button Text</a>

Button Outline

btn-outline-primary btn-outline-secondary btn-outline success btn-outline-info

btn-outline-warning btn-outline-danger btn-outline dark btn-outline-light

<a class="btn btn-outline-primary" href="#">Button Text</a> 
<a class="btn btn-outline-secondary" href="#">Button Text</a> 
<a class="btn btn-outline-success" href="#">Button Text</a> 
<a class="btn btn-outline-info" href="#">Button Text</a>
<a class="btn btn-outline-warning" href="#">Button Text</a> 
<a class="btn btn-outline-danger" href="#">Button Text</a> 
<a class="btn btn-outline-dark" href="#">Button Text</a> 
<a class="btn btn-outline-light" href="#">Button Text</a>

Button Sizes

btn-lg Default btn-sm

<a class="btn btn-primary btn-lg">Button Text</a>
<a class="btn btn-primary">Button Text</a>
<a class="btn btn-primary btn-sm">Button Text</a>

Button Block

btn-block Full-Width Button

<a class="btn btn-primary btn-block" href="#">Button Text</a>

Active / Disabled Buttons

active disabled

<a class="btn btn-primary active" href="#">Active Button Text</a>
<a class="btn btn-primary disabled" href="#">Disabled Button Text</a>

Spinning Buttons

Loading ... Loading ... Loading ...

<a class="btn btn-primary" href="#">
  <span class="spinner-border spinner-border-sm"></span>
</a>

<a class="btn btn-primary" href="#">
  <span class="spinner-border spinner-border-sm">
  Button Text
  </span>
</a>

<a class="btn btn-primary disabled" href="#">
  <span class="spinner-border spinner-border-sm">
  Button Text
  </span>
</a>

<a class="btn btn-primary disabled" href="#">
  <span class="spinner-grow spinner-grow-sm">
  Button Text
  </span>
</a>

Reference

W3Schools - BS4 Buttons