:invalid and :required CSS pseudo-classes don't work with jQuery traversing methods when selector matches multiple form elements
<p><b>HTML</b></p>

<pre><code>&lt;input required&gt;
&lt;input required&gt;
&lt;input required&gt;
</code></pre>

<p><b>JavaScript</b></p>

<pre><code>/*
// Add script for noted :invalid check to work
// :required can also be made to work by modifying script, i.e. changing "invalid" to "required" where noted
// Source: http://stackoverflow.com/a/15821264/2171842
jQuery.extend(jQuery.expr[':'], {
    invalid : function(elem, index, match){ // modify pseudo-class here
        var invalids = document.querySelectorAll(':invalid'), // modify pseudo-class here
            result = false,
            len = invalids.length;

        if (len) {
            for (var i=0; i&lt;len; i++) {
                if (elem === invalids[i]) {
                    result = true;
                    break;
                }
            }
        }
        return result;
    }
});
*/
$(document).ready(function(){
  // works
  console.log("number of invalid elements: "+$(":invalid").length); // selector matches multiple elements
  console.log("is first element invalid? "+$("input:first").is(":invalid")); // selector matches one element
  console.log("is at least one element enabled? "+$("input").is(":enabled")); // selector matches multiple elements
  // fails without additional script, :required also fails
  console.log("is at least one element invalid? "+$("input").is(":invalid")) // selector matches multiple elements
  console.log("number of invalid elements: "+$("input:first").siblings(":invalid").andSelf(":invalid").length) // selector (siblings(":invalid")) matches multiple elements
});
</code></pre>

<p>Console error: <code>Uncaught Error: Syntax error, unrecognized expression: unsupported pseudo: invalid</code>. Follow comments in code to navigate problem. Is this a bug or just an unsupported feature in jQuery?</p>

<p><b>Fiddle</b></p>

<p><a href="https://jsfiddle.net/cbqdp43s/" rel="nofollow">https://jsfiddle.net/cbqdp43s/</a></p>