CSS nesting improves with CSSNestedDeclarations

To fix some weird quirks with CSS nesting, the CSS Working Group resolved to add the CSSNestedDeclarations interface to the CSS Nesting Specification. With this addition, declarations that come after style rules no longer shift up, among some other improvements.

These changes are available in Chrome from version 130 and are ready for testing in Firefox Nightly 132 and Safari Technology Preview 204.