I knocked this Chrome extension up in a couple of hours to help me with documenting UI functionality, principally for accessibility and keyboard navigation. I get asked all the time what keyboard functionality is/should be available in mock-ups and prototypes, so I've made an extension that overlays the main navigation keys over a web page.
I recommend pinning the extension in your browser toolbar (done via the jigsaw icon, followed by the pin icon, in Chrome). From here you can click the extension's icon to reveal the show/hide keyboard button. From here the displayed keys will animate when you press them on your physical keyboard.
You can get the extension on the Chrome Web Store.
If you want the latest features before they're approved by Google, or to add your own, you can just clone this repo, hit developer mode on the Chrome extensions page and "load unpacked extension".
Please report any sites that you find this doesn't work on and I'll do my best to see if I can fix it.