Certain key combinations are impossible to press because they are reserved by the operating system (Ctrl+Alt+Del or Alt+Tab, for example) or by the web browser. If you press one of these reserved combinations, the effect will be observed locally, not remotely, and the remote desktop will receive only some of the keys.
You will have access to a built-in on-screen keyboard, which allows for keys to be sent to the remote desktop without affecting the local system. If the device you are using does not have certain keys that the remote desktop depends on, such as the arrow keys or Ctrl, you can use the on-screen keyboard for this, too. It is available by selecting the "On-screen Keyboard" option from the menu.
Clicking (or tapping) the buttons of the on-screen keyboard has the same effect as pressing the same buttons on a real keyboard, except that the operating system and browser will not intercept these keypresses; they will only be sent to the remote desktop.