ENH: Add option to highlight NaN cells
Useful for quickly inspecting data in an ipynb
- Add display.highlight_nan option (default True)
- Add argument to DataFrame.to_html
  and HTMLFormatter.__init__
- Add test

Thanks for the PR, but this solution is too specific. The actual feature needed is
conditional formatting and it needs to be solved in a more general way, one
which will support this as a private case.

closing.

related https://github.com/pydata/pandas/issues/3190?source=cc
 This is nearly exactly what I have in mind.  (I would like to de-emphasize NaN cells.)

My idea was to add a class to empty cells:

``` html
<td class="pandas-nan">
```

Then, one can use CSS for arbitrary styling.  (No, even CSS3 does not have selectors yet based on _content_ which would allow to select NaN-cells.)

Since CSS allows many other types of conditional formatting already, I would say that this is a useful addition that does not interfere with other enhancement ideas (templates, ...)?!
 I like you're solution much better @hmeine but it's for @y-p to decide.
 @hmeine @sburns We might be waiting quite a while for @y-p to decide since he's stepped out of pandas dev. I think may be too specific. We don't really want to add anymore arguments to `to_html`, though I understand the need to be able to customize HTML output. If one of you wants to take the lead on #3190, that would be most welcome.
