It says the tray is sturdy enough to hold an ironing station - Reliable does sell irons with separate tanks. Check the weight on those and add however much the water would weigh and you should have an idea of what it holds.
The legs look thinner than my Pfaff ironing board. That board was sold with a gravity feed iron and it has a fan and heater. The tray on that board held an older Rowenta steam generator and when it was full, the tank was heavy.