Currently on iOS you can only pinch to zoom in (and pinch to return to 100%). Pinching to zoom out (to less than 100%) would be helpful for all iOS devices. Some browsers allow users to zoom in/out with “Command” + “+/-“, which works fine if there’s a connected keyboard (Firefox does not seem to support zoom hotkeys for the Firefox iOS browser).
Why is it necessary? Unfortunately, some websites are not responsive, and parts get cut off when displayed on an iPad (and sometimes the keyboard or auto-suggestion bar blocks important buttons).