From the W3C wiki:
The :root
pseudo-class represents an element that is the root of the document. In HTML, this is always the HTML element.
CSS is a general purpose styling language. It can be used with other document types, not only with HTML, it can be used with SVG for example.
From the specification (emphasis mine):
This specification defines Cascading Style Sheets, level 2 revision 1 (CSS 2.1). CSS 2.1 is a style sheet language that allows authors and users to attach style (e.g., fonts and spacing) to structured documents (e.g., HTML documents and XML applications).
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