The MIDI specification specifies a 5 mA current loop, and shows three 220 Ω resistors to limit the current when used with a 5 V transmitter.
When you have a 3.3 V transmitter, the two resistors in the transmitter must be smaller to get the same current, as shown in the Electrical Specification Update.

The receiver does not need to know what voltage the transmitter uses; it always uses the same 220 Ω resistor. All transmitters are designed to work with that same receiver circuit. This means that all transmitters and receivers are compatible.

(The receiver can use either 5 V or 3.3 V as supply for the optoisolator; this part of the circuit is isolated from the current loop, and does not affect compatibility of the MIDI interface. But note that only the PC900/H11L1 optoisolators can run at 3.3 V; other common models like the 6N137 or 6N138 require 5 V.)

Thank you very much. That exactly answers my question!

I see. So I can transmit from my 5Volt keyboard to a 3.3 volt keyboard with no problems. Is that correct?

The 5 volt spec used 2 x 220 ohms
The 3.3 volt spec uses 10 & 33

That seems proportionally incorrect. Why is that?

The 5 volt spec used 2 x 220 ohms. The 3.3 volt spec uses 10 & 33

That seems proportionally incorrect.

The voltage drop over the receiver (1.1 V over the 220 Ω resistor, and somewhere between 1.1 V and 1.7 V over the optocoupler's LED) should stay constant. The transmitter's resistors must drop the remaining voltage; in the worst case, this would be about
5.0 V − 1.1 V − 1.7 V = 2.2 V, or
3.3 V − 1.1 V − 1.7 V = 0.5 V.

(The section "Low-Voltage Signaling Technical Notes" of CA33 shows how to compute the resistor values. However, in practice, designers make less pessimistic assumptions about the LED forward voltage; many 3.3 V devices actually use 2× 47 Ω in the transmitter.)