Radio/checkbox 'required' attribute in a form?

MDL Version: 1.3.0.
Browser: Google Chrome 55.0.2883.87 m
OS: Windows 10.

I'm trying to use MDL for a simple survey. List of questions with radios or checkboxes as answers. Radios/Checkboxes are not pre-checked when a page is loading and User has to check a radio or at least one checkbox for every answer.
If User didn't select the answer for at least one question, the standard notification (popup, pointing to the first radio/checkbox with a text 'Please select one of this options') should be shown when clicking on Submit button.

I'm using the required attribute for radio, but while submitting the desired popup is only 'blinking' (show and hide very fast) and not showing properly.

Simple demostration is on [codepen](http://codepen.io/nicolausYes/pen/dOBbYv).