At the moment, the placeholders look just like normal values - this is poor UX.
Whenever the span is set to contain the placeholder, add a css class to it.
Whenever the span is set to contain a normal value, remove the css class from it.
This will allow the appearance to change (easily) so that the placeholder doesn't look just like all the other values.
The default name of the css class should be 'empty', or something similar, and would allow span.empty to be styled differently from the normal styling.
The css class should be configurable using opts.placeholderClass or similar (although this is not all that important).