How to make the checkbox unchecked by default always

jQuery

$('input[type=checkbox]').removeAttr('checked');

Or

<!-- checked -->
<input type='checkbox' name='foo' value='bar' checked=''/> 

<!-- unchecked -->
<input type='checkbox' class='inputUncheck' name='foo' value='bar' checked=''/> 
<input type='checkbox' class='inputUncheck' name='foo' value='bar'/> 

+

$('input.inputUncheck').removeAttr('checked');

If you have a checkbox with an id checkbox_id.You can set its state with JS with prop('checked', false) or prop('checked', true)

 $('#checkbox_id').prop('checked', false);

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 :)