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.
The :valid CSS pseudo-class represents any <input> or <form> element whose content validates correctly according to the input's type setting. This allows to easily make valid fields adopt an appearance that helps the user confirm that their data is formatted properly.
The :visited CSS pseudo-class lets you select only links that have been visited. This style may be overridden by any other link-related pseudo-classes, that is :link, :hover, and :active, appearing in subsequent rules. In order to style appropriately links, you need to put the :visited rule after the :link rule but before the other ones, defined in the LVHA-order: :link — :visited — :hover — :active.
The <angle>CSS data type represents angle values. Positive angles represent clockwise angles, negative angles represent counterclockwise angles. Its syntax is a <number> data type immediately followed by the unit (deg, grad, rad or turn). Like for any CSS dimension, there is no space between the unit literal and the number.
The animation-play-stateCSS property determines whether an animation is running or paused. This can be queried to determine whether or not the animation is currently running. In addition, its value can be set to pause and resume playback of an animation.
The background-position-x CSS property sets the initial horizontal position, relative to the background position layer defined by background-origin for each defined background image. For more information, see the background-position property, which has been widely supported.