The default standard power limit is still the same as it ever was on each USB version. There's negotiation that needs to happen to tell the device how much power is allowed, and if you go over, I think over current protection is part of the USB spec for safety reasons. There's a bunch of different protocols, but USB always starts at 5V, and 0.1A for USB 2.0, and devices need to negotiate for more. (0.15A I think for USB 3.0 which has more conductors)
As an example, USB 2.0 can signal a charging port (5V / 1.5A max) by putting a 200 ohm resistor across the data pins.