Fix: :root no longer causes invalid selectors

[cornedor]

Description

The :root selector caused invalid selectors because the namespace was added before it. This PR handles :root the same way as html and body are handled. This is useful for defined css variables. For example:

:root {
	--color-primary: #FF0;
}

will now become:

.editor-styles-wrapper {
	--color-primary: #FF0;
}

How has this been tested?

I've build a test case and ran the tests

Screenshots

Types of changes

This is a bug fix. Without this PR :root will be ignored by the browser.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[youknowriad]

Seems reasonable to me. Thoughts @jasmussen

[jasmussen]

Seems reasonable to me as well. I've very rarely used :root, but the use case seems okay to me.

Can we think of any situations where a web developer might not want :root to be rewritten?

[cornedor]

:root might be a way to break out of the namespace, instead of replacing :root with with .editor-styles-wrapper it should not be prepended, but I think personally that it could become confusing. .editor-styles-wrapper :root is an invalid selector and does nothing.