Passing keys doesn't work

I'm using this as a testcase: https://www.w3schools.com/graphics/tryit.asp?filename=trygame_controllers_keys

The arrow keys should move the red rectangle around, but it's not working.

And if you try to edit the code to the left of the game, it seems all you can do is to delete characters with backspace or to insert line breaks with enter, normal characters don't get registered. And the changes to the text only become visible with some delay.

Do we need to use Firefox's marionette protocol to make the passing of input events truly robust? The downside would be that this would tie Browsh even more to Firefox.

Maybe all of these issues are fixable in the webextension.

One more testcase: http://www.gaminglogy.com/tutorial/controls-keyboard/demo/index.html (the sprite is supposed to get moved by the ASDW keys)

Please contribute further testcases to this bug report.