How do voltages at outputs drop as current increases?
RdsOn = ΔVo/ΔIo  ( or equivalent series resistance, ESR) for the Pch and Nch FETs

ΔVo=Vcc-Voh for "1" and ΔVo=Vol for "0"  depends on load current.

e.g. for Voh for TTL loads,  Vcc=4.5V (5V-10%)Voh=3.98V  Io =-4mA thus RdsOn=(4.5-3.98)/4mA= 520mV/4mA = 130 Ω (max) for Voh or output ="1"  (Pch FET)

e.g. for Vol for TTL loads, Vol= 0.26V/4mA = 65 Ω  (max) for Vol  or output ="0" (Nch FET)

Note that 74HCTxx series is designed to replace TTL IC's with a fanout of 10 loads so the input threshold is 1.5V instead of symmetrical Vcc/2.

74HCxxx is the standard 5V CMOS family with input threshold of Vcc/2 +/- xx%. 

Also each std CMOS families have slightly different RdsOn but tend to be symmetrical e.g. 3.3V logic 74ALCxx is closer to 25 Ω while 5V logic 74HCxx is 50 Ω and legacy CD4000 15V family was ~ 300 Ω @ 5V

Nexperia specs for 74HC08 has Vol/Iol= 37.5 Ω (typ), 65 Ω (max) @4.5V , other Mfg's may vary slightly, but these are usually standardized.