CSS root selector select root element of the document.The :root selector matches the document's root element.
The :root selector allows you to target the highest-level "parent" element in the DOM, or document tree. It is defined in the CSS Selectors Level 3 spec as a “structural pseudo-class”, meaning it is used to style content based on its relationship with parent and sibling content.
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.
In the overwhelming majority of cases you're likely to encounter, :root refers to the <html> element in a webpage. In an HTML document the html element will always be the highest-level parent, so the behaviour of :root is predictable. However, since CSS is a styling language that can be used with other document formats, such as SVG and XML, the :root pseudo-class can refer to different elements in those cases. Regardless of the markup language, :root will always select the document's top-most element in the document tree.
:root selector and html selector both target the same HTML elements, it may be useful to know that :root actually has a higher specificity. Pseudo-class selectors (but not pseudo-elements) have a specificity equal to that of a class, which is higher than a basic element selector.