       internal default value 'pd-t'.

        Returns
        -------
        Styler

        Notes
        -----
        Tooltips are created by adding `<span class="pd-t"></span>` to each data cell
        and then manipulating the table level CSS to attach pseudo hover and pseudo
        after selectors to produce the required the results.

        The default properties for the tooltip CSS class are:

        - visibility: hidden
        - position: absolute
        - z-index: 1
        - background-color: black
        - color: white
        - transform: translate(-20px, -20px)

        The property 'visibility: hidden;' is a key prerequisite to the hover
        functionality, and should always be included in any manual properties
        specification, using the ``props`` argument.

        Tooltips are not designed to be efficient, and can add large amounts of
        additional HTML for larger tables, since they also require that ``cell_ids``
        is forced to `True`.

        Examples
        --------
        Basic application

        >>> df = pd.DataFrame(data=[[0, 1], [2, 3]])
        >>> ttips = pd.DataFrame(
        ...    data=[["Min", ""], [np.nan, "Max"]], columns=df.columns, index=df.index
        ... )
        >>> s = df.style.set_tooltips(ttips).to_html()

        Optionally controlling the tooltip visual display

        >>> df.style.set_tooltips(ttips, css_class='tt-add', props=[
        ...     ('visibility', 'hidden'),
        ...     ('position', 'absolute'),
        ...     ('z-index', 1)])  # doctest: +SKIP
        >>> df.style.set_tooltips(ttips, css_class='tt-add',
        ...     props='visibility:hidden; position:absolute; z-index:1;')
        ... # doctest: +SKIP
        z1Tooltips can only render with 'cell_ids' is True.z=Tooltips render only if `ttips` has unique index and columns.N)