Jonathan Neal wrote a neat @scope polyfill for this CodePen. The problem is it only applies to the explicit @scope (start) to (end) form, and relies on a strange custom syntax – not really a polyfill in the strictest sense then. Everything else I could find was implementing the outdated scoped attribute specification.
My main concern is implicit start scopes via <style> position, and I want it to work on spec-compliant code (like a real polyfill!) so that's what this aims to be, using Jonathans same principle strategies: building positional identity selectors of the form :where(:nth-child(x) > :nth-child(y) etc...) to apply scoped rules, and MutationObserver to respond to changes in the DOM and update the selectors accordingly.