social/mail_digest/templates/digest_layout_preview.xml

<?xml version="1.0" encoding="UTF-8"?>
<odoo>

  <template id="digest_layout_preview" name="Mail digest layout preview">
    <style>
      #digest_layout_wrapper {
        max-width: 600px;
        margin: 0 auto;
        border: 1px dashed #ddd;
        padding: 4em;
      }
    </style>
    <div id="digest_layout_wrapper">
      <t t-raw="digest_html" />
    </div>
  </template>

</odoo>