What happens if the input high and input low of Buf2 become the same?
It seems you are asking about differences in the definitions of the high and low for output and input.

The point is that Buf2 must have its input signal in its input voltage defined space to operate properly. Your picture shows it will treat low for volages &lt;0.3, and high for voltages >0.7. Thus previous output device Buf1 must output within these ranges. Rage of [0.3;0.7] will not guarantee Buf2 to operate properly (whatever fault it may cause to the Buf2 or its output level).

Thus as drawn on your picture - the circuit will work properly, because Buf1 outputs low &lt;0.15 which is &lt;0.3, and high >0.8 which is >0.7.

In general you should always refer to the datasheet of the devices being connected. Digital logic used to have their output levels defined explicitly so that you can match the standards and see the noise slack you may have.