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-system-metric(scrollbar-thumb-proportional)CSSpseudo-class will match an element if the computer's user interface uses proportional scrollbar thumbs; that is, the draggable thumb on the scrollbar resizes to indicate the relative size of the visible area of the document.
The :-moz-system-metric(windows-default-theme) CSS pseudo-class matches an element if the user is currently using one of the following themes in Windows: Luna, Royale, Zune, or Aero (i.e., Vista Basic, Vista Standard, or Aero Glass). This will exclude Windows Classic themes as well as third-party themes.
The negation CSS pseudo-class, :not(X), is a functional notation taking a simple selector X as an argument. It matches an element that is not represented by the argument. X must not contain another negation selector.