Fixes some browsers allowing invalid forms to submit.

form-validation-buggyfill.js

// Fixes some browsers allowing invalid forms to submit, mostly an issue with Safari allowing empty required fields.
// http://caniuse.com/#search=required
window.addEventListener('submit', (event) => {
  if (!event.target.noValidate && !event.target.checkValidity()) {
    event.preventDefault()
    window.alert('Please correct invalid form fields and resubmit.')
  }
})

This has since been fleshed out, given it's own GitHub repo and has been published to NPM.