Improve keyboard accessibility

[PolariTOON]

This PR replace the dl elements with details elements, making tests expandable with the keyboard.

Then it makes spec and mdn links inside tests focusable with the keyboard.

It also displays a visual hint when focusable elements (e.g. links or summary elements) have the focus: a background color change or an outline.

This closes #205.

[LeaVerou]

Thank you!! Could you post a screenshot please?

[PolariTOON]

Sure!

All taken in Firefox Nightly (it looks the same in Chrome Dev and Epiphany TP):

• summary element focused (in an open details):

• A spec (or MDN) link focused:

• A link in the spec list focused:

• Your logo focused:

[LeaVerou]

Cool, thanks! Merged.