There are usually multiple sources, and each might be nested in a `VideoObject`. Check the source of [mozilla.jp/videos](http://www.mozilla.jp/videos/). In this case, the markup can be:
```python
{% if ft in videos %}
  <span itemprop="encoding" itemscope itemtype="http://schema.org/VideoObject">
    <meta itemprop="encodingFormat" content="{{ ft }}">
    <source itemprop="contentURL" src="{{ videos[ft] }}" type="{{ mime[ft] }}">
  </span>
{% endif %}
```