I was watching a video coding html, and I saw in the video they used a bit of code that looked like this:
<body>
{% block %}
{% endblock %}
</body>
Is this some form of commenting in html? I think the coder they were using was Espresso for Mac. I've just never seen it. The video is here: http://vimeo.com/7405114, at about 5 minutes in, toward the very end.
They are Django template blocks:
http://docs.djangoproject.com/en/dev/ref/templates/builtins/
Those are a Django block tag. They're used in template inheritance to define a block of text in a template that can be replaced in children templates.
Comments in HTML are created by <!-- and -->. The {% ... %} is propably a placeholder which gets replaced later.
If you look closely in the video it says that they are blocks for the Django template system.
They contain code in the Django template language.