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A touchpad or trackpad is a pointing device featuring a tactile sensor.
I want to get tap to click work with FocalTechPS touchpad of ASUS laptop.
Other natural features later: double-tap, ...
In Fig. 1, you see that the system does not even notice Touchpad drivers, compare to Fig. 2 in the differential solution with Wayland.

Benchmark of the touchpad: one found in OS X in Mac devices since last five years

Fig. 1 Settings of Mouse and Touchpad in the OS when in X11

Doing egrep -i 'synap|alps|etps|elan' /proc/bus/input/devices based on the SynapticsTouchpad Debian documentation gives blank. Going the file manually through, I find the following.

I: Bus=0011 Vendor=0002 Product=0012 Version=0000
N: Name="FocalTechPS/2 FocalTech Touchpad"
P: Phys=isa0060/serio4/input0
S: Sysfs=/devices/platform/i8042/serio4/input/input12
U: Uniq=
H: Handlers=mouse0 event5 
B: PROP=5
B: EV=b
B: KEY=e520 10000 0 0 0 0
B: ABS=260800010000003

Maybe, related here.
Bugs related in the application of the reverse engineered approaches here.
No integration in the kernel has been done yet, etc discussed here.
No focaltech-dkms exists anymore, and not maintained.

Hardware: Asus Zenbook UX303UA R4028T

Touchpad: FocalTechPS

OS: Debian 9.1

Window manager: Gnome 3.22 on X11

Packages: xserver-xorg-input-libinput

Debian docs: SynapticsTouchpad

Related dev: Arch Linux here, Ubuntu 15.04 here, private Github of mgottschlag here but based on some version of focaltech.h headers not found anywhere

Move away from X11 and start to use Wayland.
Tap to click work there.
Restart and Login Gnome with Wayland.
You see there are significantly more features supported in Wayland of Fig. 2 than X11 of Fig. 1.
Differences in usage

• two finger scrolling is opposite now than previously in X11, also shown as reseted Natural Scrolling setting. Set Natural scrolling of Fig. 2 to off to have equivalent settings as in Fig. 1.

Fig. 2 Debian Settings > Mouse and Touchpad in Wayland