Disallow custom properties outside of :root selectors.
a { --foo-bar: 1px; }
/** ↑ ↑
* These selectors and these types of custom properties */The following patterns are considered warnings:
a { --foo-bar: 1px; }:root, a { --foo-bar: 1px; }The following patterns are not considered warnings:
:root { --foo-bar: 1px; }