This is a feature request for blocks which are required to be implemented by the leaf template. One example of where this would be useful is in title blocks. In the parent template:

{% block title required %}{% endblock %}

And in the child:

{% extends "example.html" %}
{% block title %}
<title>My Page</title>
{% endblock %}

If the title block is not present in the latter, an error is rased. If the block has any contents other than whitespace or comments in the former, an error is raised. In effect, this requires all pages to have titles.