Selector pseudo-class :root

The :root CSS pseudo-class matches the root
element of a tree representing the document. In
HTML, :root represents the element and is
identical to the selector html, except that its
specificity is higher.

:root can be useful for declaring global CSS
variables:

:root {
  --main-color: hotpink;
  --pane-padding: 5px 42px;
}