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.
You know, I just noticed that my code doesn't behave very well with multi-line summaries. Maybe that's what you're talking about. Let me make a few tweaks.