The non-standard proprietary :-ms-input-placeholderpseudo-class represents the placeholder text of a form element. This allows web developers and theme designers to customize the appearance of placeholder text. This pseudo-class is only supported by Internet Explorer and Microsoft Edge.
The :fullscreen CSS pseudo-class applies to any element that's currently being displayed in full-screen mode. It selects not only to the top level element, but to the whole stack of elements that appears.
The :hover CSS pseudo-class matches when the user designates an element with a pointing device, but does not necessarily activate it. This style may be overridden by any other link-related pseudo-classes, that is :link, :visited, and :active, appearing in subsequent rules. In order to style appropriately links, you need to put the :hover rule after the :link and :visited rules but before the :active one, as defined by the LVHA-order: :link — :visited — :hover — :active.
The :invalid CSS pseudo-class represents any <input> or <form> element whose content fails to validate according to the input's type setting. This allows you to easily have invalid fields adopt an appearance that helps the user identify and correct errors.
The :link CSS pseudo-class lets you select links inside elements. This will select any link which has not yet been visited, even those already styled using selector with other link-related pseudo-classes like :hover, :active or :visited. In order to appropriately style links, you need to put the :link rule before the other ones, as defined by the LVHA-order: :link — :visited — :hover — :active. The :focus pseudo-class is usually placed right before or right after :hover, depending on the expected effect.
The :-moz-full-screen-ancestorCSSpseudo-class applies to all ancestors of the full-screen element, except containing frames in parent documents, which are the full-screen element in their own documents. However, those elements' ancestors have this class applied to them.
The :-moz-locale-dir(ltr)CSSpseudo-class matches an element if the user interface is being displayed left-to-right. This is determined by the preference intl.uidirection.locale (where locale is the current locale) being set to "ltr".
The :-moz-only-whitespaceCSSpseudo-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.