title: Extending the HTML 5/JavaScript input validation API
body: <p>I'm to work with with the HTML 5 validation on an input type number field, that uses min and step attributes, and an initial value, i.e.,</p>
  
  <pre><code>&lt;input type="number" min="50" step="50" value="100" /&gt;
  </code></pre>
  
  <p>However, I want the validation to allow the user to enter values that are greater than the min, but between the steps, here shown in bold:</p>
  
  <p>&nbsp;&nbsp;&nbsp;&nbsp;50, <strong>51 ... 99</strong>, 100, <strong>101 ... 149</strong>, 150, <strong>151 ...</strong></p>
  
  <p>Can I somehow 'hook' into the validation API after it has performed its work, and if the only error that it finds is a step error, reset the valid condition so that the form that it the field is in can be submitted?</p>
  
  <p>Sure would be nice if besides a message string parameter, the <code>element.setCustomValidity()</code> function could also accept a callback function that returned a empty string if the element's value passed the check(s) in the callback function or return a non-empty message string if the value didn't pass.</p>