Use YUI 3.10 (now that conflict with ASCIIMathML is solved).
{% extends "site/layout/base.html" %}
{% load i18n %}
{% block content %}
<h1>404: {% blocktrans %}Page not found{% endblocktrans %}</h1>
{% blocktrans %}The requested page could not be found.{% endblocktrans %}
{% endblock %}