Open the game and follow the mouse with your eyes while you click each 8-directional edge of the game's viewport and then click the center.
Now you can let go of the mouse (don't need to move it out of the game) and the game should follow your gaze.
This demo is impressive. It's accurate enough for me to isolate the orange ball from the cluster.
On a website, the calibration could be done in a modal overlay. "Look at this dot while clicking it wherever it appears". After then Nth click, the modal goes away and the user lands on the website ready to gaze.