What instruments made up a typical panel of a WWI airplane?
Did any country regulate the required cockpit instruments prior to the United States "Air Commerce Act" of 1926?
If you are asking about military aircraft there would often have been a compass, oil pressure gauge, a not very sensitive altimeter, a tachometer and some sort of airspeed indicator. Not sure about fuel gauges. I have also seen bank indicators. There was not the standards for instruments as in later days and some items were specific to individual aircraft types. Some of these guys could fly a sort of instrument flight but had not much in the way of navigation aids.
Eyes and ears(including the inner ear) were the only "standard" instruments at that time. Although some crude gauges were in moderate use.(not all were found in all aircraft and some aircraft had few or none) DaveK listed most of them. A map and some navigation tools would also be fairly common, especially on longer flights. Fuel gauges were mostly of the sight glass type. (clear tube to physically see the fuel level)