the current output stream - start with the main text stream

Is the UI running in HTML mode? This tells us if we have a full HTML UI or a text-only UI. Full HTML mode applies if we're running on a Multimedia TADS interpreter, or we're using the Web UI, which runs in a separate browser and is thus inherently HTML-capable.

(The result can't change during a session, since it's a function of the game and interpreter capabilities, so we store the result on the first evaluation to avoid having to recompute it on each query. Since 'self' is a static object, we'll recompute this each time we run the program, which is important because we could save the game on one interpreter and resume the session on a different interpreter with different capabilities.)

Switch to a new active output stream. Returns the previously active output stream, so that the caller can easily restore the old output stream if the new output stream is to be established only for a specific duration.

run the given function, using the given output stream as the active default output stream