Help to understand output levels of clock buffer
A buffer for logic signals is very different from an analog amplifier.

With a logic buffer, you don't want the output voltage to depend on the input voltage; instead, you want it to output a logic high when the input is a logic high, and output a low when the input is low.

Therefore, the input specifications for the buffer indicate what voltage levels it considers to be high and low, and the output specifications indicate the capabilities of its drive circuit to output high and low under various load conditions.