I have added the wait to prevent a race case on Desktop b2g. By running the keyboard.py I've notice that we try to tap the keys right after tapping Alt or Numeric key, I believe this is because python runs asynchronously. Basically what this code does is wait for the element to be visible before tapping, without it we would fail when trying to send a string to keyboard that has more than on type of chars ( example: "aB#45_Ca")