social/mail_browser_view/demo/mail.xml

<?xml version="1.0"?>
<odoo>
  <data>
    <record id="browser_view_demo" model="mail.mail">
      <field name="body_html">
<![CDATA[
<html>
  <body>
    <p>Here is placeholder example:</p>
    <a href="#" class="view_in_browser_url">View this mail in browser</a>
    <p>For everything else, there is <a href="https://www.google.com">Google</a></p>
  <body>
</html>
]]>
      </field>
    </record>
  </data>
</odoo>