The assertion that the fundamental signals, Red Green Blue, H-Sync and V-Sync are the same is quite true. BUT...
The Quadra 840AV ( and every Beige Mac) wants to read the ID bits on pins 4, 7, and 10 to determine which genuine Mac display is attached. Those pins all disconnected yields a code for "No display Attached", and you will get no picture.
If you can find an adapter that claims to be a MAC adapter, or one with DIP switches, that will be superior. The Dip switches allow multiple combinations of pins 4, 7 and 10 interconnections (some through diodes in the adapter) to yield many Combinations of ID bits. Since most of these are obsolete, the ones that claim to be a Mac adapter provide the code for the largest Multi-Sync screen and seem to work OK.