Are you saying that you can't get the title of the active window in X11 without using some features specific to the X.Org implementation?
It looks like the core X11 protocol spec [1] defines all that's needed, specifically the GetInputFocus, QueryTree and GetProperty messages. You might also want some things from the EWMH spec [2] (e.g. _NET_WM_NAME for UTF-8 or _NET_WM_WINDOW_TYPE to identify top-level application windows) but none of this seems like an implementation-specific X.Org feature.
It looks like the core X11 protocol spec [1] defines all that's needed, specifically the GetInputFocus, QueryTree and GetProperty messages. You might also want some things from the EWMH spec [2] (e.g. _NET_WM_NAME for UTF-8 or _NET_WM_WINDOW_TYPE to identify top-level application windows) but none of this seems like an implementation-specific X.Org feature.
[1] https://x.org/releases/X11R7.7/doc/xproto/x11protocol.html
[2] https://specifications.freedesktop.org/wm/latest/