This is a feature test and polyfill for the HTML5 details/summary markup pattern, with a couple of extras:

• Allows you to expand and collapse the details content above/below specified screen sizes
• Adds keyboard navigation and toggling
• Adds a bunch of ARIA goodness

It should be noted that mathiasbynens has an especially robust jQuery-based test/polyfill for this markup, as well.

Updates to come, and here is a demo.