Uncaught TypeError: Cannot read property 'getElementsByTagName' of null
Move the <script> from the <head> element to the <body> element.
document.body won't get a value assigned to it until the body element has been created.
Since you are trying to get all the elements in the <body>, you'd want to be at the end of the body element. Or put it in a function and call it from an event that fires after the body has been created (such as the window load event). To read more about that try, https://developer.mozilla.org/en-US/docs/Web/API/Window/load_event.