How to make the checkbox unchecked by default always

No, there is no way in simple HTML. Javascript might be your only solution at this time..

Loop through all inputs in javascript, check if they’re indeed a checkbox and set them to unchecked:

var inputs = document.getElementsByTagName('input');

for (var i=0; i<inputs.length; i++)  {
  if (inputs[i].type == 'checkbox')   {
    inputs[i].checked = false;
  }
}

wrap it up in a onload listener and you should be fine then 🙂