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 :out-of-range CSS pseudo-class matches when an element has its value attribute outside the specified range limitations for this element. It allows the page to give a feedback that the value currently defined using the element is outside the range limits. A value can be outside of a range if it is either smaller or larger than maximum and minimum set values.
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 :any-linkCSSpseudo-class represents an element that acts as the source anchor of a hyperlink independent of whether it has been visited, that is, it matches every <a>, <area> or <link> elements with an href attribute. So, it matches all elements that match :link or :visited.
The :-moz-placeholderpseudo-class represents any form element displaying placeholder text. This allows web developers and theme designers to customize the appearance of placeholder text, which is a light grey color by default. This may not work well if you've changed the background color of your form fields to be a similar color, for example, so you can use this pseudo-class to change the placeholder text color.
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.