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 unicode-range CSS descriptor sets the specific range of characters to be used from a font defined by @font-face and made available for use on the current page. If the page doesn't use any character in this range, the font is not downloaded; if it uses at least one, the whole font is downloaded.
The @supportsCSSat-rule associates a set of nested statements, in a CSS block, that is delimited by curly braces, with a condition consisting of testing of CSS declarations, that is property-value pairs, combined with arbitrary conjunctions, disjunctions, and negations of them. Such a condition is called a supports condition.
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 contain property allows an author to indicate that an element and its contents are, as much as possible, independent of the rest of the document tree. This allows the browser to recalculate layout, style, paint, size, or any combination of them for a limited area of the DOM and not the entire page. This property is useful on pages that contain a lot of widgets that are all independent as it can be used to prevent one widget's CSS rules from changing other things on the page.
CSS counters are, in essence, variables maintained by CSS whose values may be incremented by CSS rules to track how many times they're used. This lets you adjust the appearance of content based on its placement in the document. CSS counters are an implementation of Automatic counters and numbering in CSS 2.1.
The element()CSS function defines an <image> value generated from an arbitrary HTML element. This image is live, meaning that if the HTML element is changed, the CSS properties using the resulting value are automatically updated.
The <frequency>CSS data type denotes a frequency dimension, like the pitch of a speaking voice. It consists of a <number> immediately followed by the unit. Like for any CSS dimension, there is no space between the unit literal and the number.