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.
:-moz-loading CSS pseudo-class matches elements, which can't be displayed, because they have not started loading, such as images that haven't started to arrive yet. Note that images that are in the process of loading are not matched by this pseudo-class.
This is primarily useful for theme developers.