Why would you need to manipulate the shadow DOM? What do you mean by "keys"? If you want to add subtitles, that is already supported. The epub3 test epub shows them being used.

The word "keys" instead of button was an unfortunate choice of mine in the context of the question. :smack:

As a non-native speaker and writer, there are times when I like to turn the subtitles on or off. There is indeed a button for that automatically appears in non-full screen mode if a valid subtitle file is present.

But .... once you switch to full screen, that button for turning the subtitles on or off is unfortunately no longer displayed.
To add those, and other buttons, you must have access to the shadow DOM.