CSS fixes for details/summary polyfills and alerts

These fixes normalize a lot of the styling between native and polyfill implementations of details/summary elements.

It also improves the layout of alerts when combined with details/summary.

Plus, some RTL enhancements.
I'm just asking if in your testing if the end result for both cases is that margin-right is set to auto. Don't know if having a non-auto margin-left would cause future issues.