CSS Code
Output
<!DOCTYPE html>
<html>
<head>
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css" integrity="sha384-1q8mTJOASx8j1Au+a5WDVnPi2lkFfwwEAa8hDDdjZlpLegxhjVME1fgjWPGmkzs7" crossorigin="anonymous">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">    

</head>

<body>
<div class="container">
<h1>HTML buttons by Bootstrap CSS classes</h1>
<p>Normal Size</p>
<p>
  <button type="button" class="btn btn-primary">Primary style</button>
  <button type="button" class="btn btn-danger">danger style</button>
  <button type="button" class="btn btn-warning">warning style</button>
  <button type="button" class="btn btn-info">info style</button>
  <button type="button" class="btn btn-success">success style</button>
  <button type="button" class="btn btn-default">default style</button>
</p>


   </div> 
</body>
</html>
HTML buttons by Bootstrap CSS classes

Normal Size