required

• Specify required='required' on an input element to prevent the form from being submitted without filling in this value.

• Redundant with pattern or type that does not allow empty string

<form>
<p>
  <label>Card number: 
    <input type="text" name="cardnumber" 
         required="required"
         pattern='\d\d\d\d ?\d\d\d\d ?\d\d\d\d ?\d(\d\d\d)?'/>
  </label>
</p>
<p>
  <label>
    <input type="submit" value="Send data" />
  </label>
</p>
</form>