Not exactly with HTML5 validation but a little JavaScript can resolve the issue, follow the example below:
<p>Password:</p>
<input name="password" required="required" type="password" id="password" />
<p>Confirm Password:</p>
<input name="password_confirm" required="required" type="password" id="password_confirm" oninput="check(this)" />
<script language="javascript" type="text/javascript">
function check(input) {
if (input.value != document.getElementById('password').value) {
input.setCustomValidity('Password Must be Matching.');
} else {
// input is valid -- reset the error message
input.setCustomValidity('');
}
}
</script>
<br /><br />
<input type="submit" />