In all, quite impressive code, here are some pointers:

• Besides the suggestion to compare with 0 via ===, JsHint could not find anything

• Consider using addEventListener instead of the old skool onxxx

• I would put preview(); and createURL(); together instead of having them in the middle of event handler assignments, either right after the var assignments or at the very bottom.

• Or you could even consider faking a click on 'resetter' which does all that

• Ternary can be your friend here:

     if (resizerVal == 0) {
         editor.className = 'minSize';
     } else {
         editor.className = '';
     }
  

  can be

     editor.className = resizerVal ? '' : 'minSize';
  

In all, quite impressive code, here are some pointers:

• Besides the suggestion to compare with 0 via ===, JsHint could not find anything

• Consider using addEventListener instead of the old skool onxxx

• I would put preview(); and createURL(); together instead of having them in the middle of event handler assignments, either right after the var assignments or at the very bottom.

• Or you could even consider faking a click on 'resetter' which does all that, if you dont use addEventListener, you could simply do

    document.getElementById('resetter').onclick();  
  

• Ternary can be your friend here:

     if (resizerVal == 0) {
         editor.className = 'minSize';
     } else {
         editor.className = '';
     }
  

  can be

     editor.className = resizerVal ? '' : 'minSize';