<form action="" method="get">
<input type="text" oninput="validateFeeling(this)" required />
<input type="submit"  value="ok" />
</form>

<script>
 function validateFeeling(input) {
   if (input.value == " ") {
     input.setCustomValidity('此項為必填');
     return false;
   } 
 }
</script>
 

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