 Used to convert a t-field specification into an output HTML field.

    :meth:`~.to_html` is the entry point of this conversion from QWeb, it:

    * converts the record value to html using :meth:`~.record_to_html`
    * generates the metadata attributes (``data-oe-``) to set on the root
      result node
    * generates the root result node itself through :meth:`~.render_element`
    