First impression is that this is a lot like their PHP language extension XHP[1], which allows you to embed XML syntax into PHP and automatically escapes variables depending on their context (inside or outside the XML):

  <?php
  // note: includes omitted
  if ($_POST['name']) {
    echo <span>Hello, {$_POST['name']}</span>;
  } else {
    echo
      <form method="post">
        What is your name?<br />
        <input type="text" name="name" />
        <input type="submit" />
      </form>;
  }

in that snippet, the $_POST variable is correctly escaped to mitigate XSS attacks. XHP also allows you to condense complex components into a single tag.

1: https://www.facebook.com/note.php?note_id=294003943919