Other than the _NET_WM_ (EWMH) and the WM_ (ICCCM) hints, there are some hints used by a single wm (such as PEKWM's _PEKWM_FRAME_ID, to handle window tabbing in this wm) and others that multiple wms agree on using (such as ye old motif hints and the pre-gnome3 gnome hints (most wms use those for compatibility purposes, though)).
Do you know other set of hints that at least two different window manager agree to use?

I want to implement tabs in my wm, but I want to conform to the way other wms do.
Also, I discovered that motif hints use some atoms to specify the menu items that operate on X clients, I will also implement that on xmenu/pmenu (so the menu can communicate directly with the window manager) (TWM and CTWM seem to use those hints).

This is the most everything-compliant wm I have found: https://github.com/bbidulock/etwm/blob/master/ewmh.c