Why is document.getElementById() not working?

Wanna test this with some fields of the editor

In my opinion, it's good that it doesn't work - because the code placed in the browser could be used to perform dangerous activities.

but there is a way around this - in javascript.

get_canvas = function()
  system.javascript("""
    global.canvas = window.player.runtime.screen.canvas
    global.context = global.canvas.getContext("2d")
  """)  
end

if you want to modify MicroStudio - the sources are on GitHub, it will be easier.