How Twitter configures a tweet button with only markup

gistfile1.html

<script src="//platform.twitter.com/widgets.js" type="text/javascript"></script>
<div>
   <a href="https://twitter.com/share" class="twitter-share-button"
      data-url="https://dev.twitter.com/pages/tweet_button"
      data-via="your_screen_name"
      data-text="Checking out this page about Tweet Buttons"
      data-related="anywhere:The Javascript API"
      data-count="vertical">Tweet</a>
</div>