The :-moz-ui-invalid CSS pseudo-class represents any validated form element whose value isn't valid based on their validation constraints, in certain circumstances. This pseudo-class is applied according to the following rules:
The :-moz-user-disabledCSSpseudo-class matches elements representing images that were not loaded because images have been entirely disabled by the user's preferences.
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.
The :nth-last-child(an+b) CSS pseudo-class matches an element that has an+b-1 siblings after it in the document tree, for a given positive or zero value for n, and has a parent element.
The :only-child CSS pseudo-class represents any element which is the only child of its parent. This is the same as :first-child:last-child or :nth-child(1):nth-last-child(1), but with a lower specificity.
The :optional CSS pseudo-class represents any <input> or <textarea> element that does not have the required attribute set on it. This allows forms to easily indicate optional fields, and to style them accordingly.
The :required CSS pseudo-class represents any <input> element that has the required attribute set on it. This allows forms to easily indicate which fields must have valid data before the form can be submitted.
The :root CSS pseudo-class matches the root element of a tree representing the document. Applied to HTML, :root represents the <html> element and is identical to the selector html, except that its specificity is higher.