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Thanks for asking.
I don't think this is possible - for technical reasons.
Javascript is executed in the respective browser's context and I
wouldn't know of a way to inject Javascript into a running web app
from outside.
What would be a possibility, though:
If that web app reacts to key presses (f.ex. + / - for channel /
volume change), you could have RB emulate keypresses and send them to
the browser.
If the web app does not support key presses for navigation, yet, and
you know who is writing that software, don't hesitate to contact them
and ask them, if they could add that feature. It should be a rather
easy to add and certainly also of great use beyond just your case.
Best regards,
Felix Schwarz