<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def start_container(self, cls, **kwargs):
 """Append a new container."""

        # Convert stylename strings to actual style elements.
        kwargs = self._replace_stylename(kwargs)
        # Create the container.
        container = cls(**kwargs)
        self._containers.append(container)