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.
Never mind, I noticed the difference now (RTL) but does the margin-left need to be set to 0 (or auto) or is it fine just setting the margin-right?