:active - A button that is being pressed down.

:after - Creates an element on the parent element

:before - Creates an element on the parent element

:empty - Any element that has no children.

:first-child - The element is the first element among a group of sibling elements.

:focus - An element with input focus.

:hover - Any element with the mouse cursor hovering over it.

:intro - Present on all elements for their first frame. Useful to start CSS transitions on creation.

:last-child - The element is the last element among a group of sibling elements.

No pseudo-class.

:only-child - The element is the only child of their parent element.

:outro - The element has been marked for deletion, and will be deleted once all CSS transitions on it has stopped.
Transitions can be started here to gracefully remove the element visually.

Unused.