Edit in JSFiddle

<form>
  Form 1
  <input type='text' oninvalid="setCustomValidity('Cusotm Alert 1');" onchange="setCustomValidity('');" placeholder='This input is required' required/>
  <input type='text' />
  <input type='submit' />
</form>
<form>
  Form 2
  <input type='text' oninvalid="setCustomValidity('Cusotm Alert 2');" oninput="setCustomValidity('');" placeholder='This input is required' required/>
  <input type='text' />
  <input type='submit' />
</form>