::selection ans ::backdrop does not inherit from root. So :root does not guarantee the CSS variable. The html selector does not solve this.

There is currently no global context in CSS, but root is the best workaround currently.

This is going to catch a lot of people off-guard because I, like many others, expect CSS Custom properties defined on :root to just be available everywhere.

The following discussion is there https://github.com/w3c/csswg-drafts/issues/6641