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.
Summary
The :-moz-only-whitespace CSS pseudo-class matches an element that has no child nodes at all or empty text nodes or text nodes that have only white-space in them. Only when there are element nodes or text nodes with one or more characters inside the element, the element doesn't match this pseudo-class anymore.
Syntax
span:-moz-only-whitespace { style properties }
Example
CSS
span:-moz-only-whitespace::before {
background-color: lime;
}
HTML
<span> </span>