Notice how the masthead at the top looks darker- those with a protanopic defecit cannot see red, and this means that the visible spectrum is narrower for them than for any other colour defective- some purely red lights may not be visible at all. This is something they have to account for when driving, as tail lights and red traffic signals are less visible to them.
Ishihara Colour Vision testing simulation
This is a simulation of the most severe form of a red defect, some with a very mild defect would see the site almost the same as a colour normal.
These are only simulations! Your monitor calibration, room lighting and own colour vision will affect what you see, but it does give a fair simulation.