Non-standard
This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
The :-moz-only-whitespace
CSS pseudo-class is a Mozilla extension that matches elements that have no child nodes at all, or only have empty text nodes or text nodes that have only whitespace in them.
Syntax
:-moz-only-whitespace
Example
HTML
<span> </span>
CSS
span:-moz-only-whitespace::before { background-color: lime; }