When I cannot zoom a website, there is usually an option in the browser to be able to do it.
On windows chrome browser game page is always fits to visible part of
the window, so zoom doing nothing.
But on iPad it seems there is bug with upper side, address line is not recognized as board edge and drawing in front of it.
Did you try to rotate screen?
Or is there option to reduce width of the window?
With opera, I select desk-view then I zoom with my 2 fingers (Android phone)