HTML rules says div needs to have it's own closing tag:

https://stackoverflow.com/questions/3558119/are-non-void-self-closing-tags-valid-in-html5

layout/blog.j2

@@ -22,7 +22,7 @@
 {% endfor %}
 </ul>
 {% endif %}
-<div class="clear"/>
+<div class="clear"></div>
 <section>
 {% filter markdown|typogrify|rellinktoabs -%}
 {% block post -%}{%- endblock %}