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>