Link to be turned into a button:

  <a class="MyButton" href="/my_page.php">My Button Text</a>

CSS

a.MyButton, a.MyButton:hover, a.MyButton:visited {
  background-color: #F8F8F8;
  border: 1px solid #e6e6e6;
  border-radius: 4px 4px 4px 4px;
  display: inline-block;    /*inline-block so width will auto expand if needed*/
  min-height: 36px;
  min-width: 100px;
  padding-top: 6px;
  padding-right: 6px;
  padding-left: 6px;
  padding-bottom: 6px;
  margin-top: 8px;
  margin-right: 8px;
  margin-left: 0px;
  margin-bottom: 8px;
  font-size: 13px;
  font-weight: normal;
  line-height: 16px;
  color: #444444;
  text-align: center;
  text-decoration: none;
  cursor: pointer;
}

Align button left / right / center

margin: auto; will not work with an inline-block.

You need to set the text alignment of the containing element to set the alignment, e.g.

$HtmlOutput .= '<div style="text-align:center;"><a class="MyButton" href="/something.php">MY BUTTON TEXT</a></div>

Centre align a group of buttons

Put the buttons inside: <div class=”MyButtonContainer”></div>

.MyButtonContainer {
  margin-top: 10px;
  margin-bottom: 10px;
  
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
}