Buildbot uses Jinja2 to render its web interface. The authoritative source for this templating engine is its own documentation, of course, but a few notes are in order for those who are making only minor modifications.
Jinja directives are enclosed in {% .. %}, and sometimes also have
dashes. These dashes strip whitespace in the output. For example:
{% for entry in entries %}
<li>{{ entry }}</li>
{% endfor %}
will produce output with too much whitespace:
<li>pigs</li>
<li>cows</li>
But adding the dashes will collapse that whitespace completely:
{% for entry in entries -%}
<li>{{ entry }}</li>
{%- endfor %}
yields
<li>pigs</li><li>cows</li>