Revisiting this. According to the spec boolean attributes should not be rendered: http://www.w3.org/html/wg/drafts/html/master/infrastructure.html#boolean-attributes

In this case I could check the value and only merge it in if not false (not sure about empty / null values). Alternatively this could be addressed in the `widget_container_attributes` block by adding an additional conditional:

```twig
{% block widget_container_attributes %}
{% spaceless %}
    {% if id is not empty %}id="{{ id }}" {% endif %}
    {% for attrname, attrvalue in attr %}{% if attrvalue != false %}{{ attrname }}="{{ attrvalue }}" {% endif %}{% endfor %}
{% endspaceless %}
{% endblock widget_container_attributes %}
```

Would updating the `widget_container_attributes` block be considered a BC break?