Firmware: GRBL 1.1

Update your Firmware!

GRBL v1.1e (and later) is mandatory and can be found here: https://github.com/gnea/grbl/releases.
For flashing Grbl with the Arduino IDE (recommended) goto: https://github.com/gnea/grbl/wiki/Compiling-Grbl.

If your machine has homing switches, you probably need to change some settings in config.h and recompile Grbl.
For details on homing cycle see:

• https://github.com/gnea/grbl/wiki/Grbl-v1.1-Configuration#22---homing-cycle-boolean
• https://github.com/gnea/grbl/wiki/Frequently-Asked-Questions#why-is-grbl-in-all-negative-coordinates-after-homing-or-it-so-annoying-and-not-what-im-used-to

Start the backend

Execute node server.js to start the backend (server).

Run LaserWeb

Open the address localhost:8000 in Google Chrome (or Chromium).

Configure Grbl settings

By typing into the command line (bottom right) you can send manual commands to GRBL.
Send the following commands:

$10=0
$30=1000   ;max. S-value for Laser-PWM
$31=0      ;min. S-value
$32=1      ;Laser Mode on
$100=160   ;steps/mm in X, depending on your pulleys and microsteps
$101=160   ;steps/mm in Y, depending on your pulleys and microsteps
$110=10000 ;max. rate mm/min in X, depending on your system
$111=10000 ;max. rate mm/min in Y, depending on your system
$120=1000  ;acceleration mm/s^2 in X, depending on your system
$121=1000  ;acceleration mm/s^2 in Y, depending on your system
$130=400   ;max. travel mm in X, depending on your system
$131=300   ;max. travel mm in Y, depending on your system
$$         ;to check the actual settings

(see https://github.com/gnea/grbl/blob/master/doc/markdown/settings.md for more details)

Configure LaserWeb

The shown examples under Settings GCode should not be blindly followed for Grbl. Please find your own settings.

Mostly working settings:

Start G-Code:        $X and M4S0 (each on one line) (nb enable laser mode in grbl config else m04 doesnt work) 
Laser On Command:    empty
Laser Off Command:   empty
PWM max S value:     1000 (depending on $30 setting of Grbl)
Homing sequence:     $H
End G-Code:          M5S0 and G0X0Y0 (each on one line)
Travel Moves (mm/s): 30 (depending on your machine)
LaserTest Power:     S value for test shot (depending on your laser and usage)
LaserTest Duration:  duration in ms (or 0 for manual toggle on/off)