Using new CSS 4 selector: user-invalid

[mfranzke]

Evaluate to use :user-invalid instead of :invalid within our code base, as this is actually the expected behaviour by our creative colleagues, as soon as it lands and is stable in the evergreen browsers:

• https://developer.mozilla.org/en-US/docs/Web/CSS/:user-invalid
• https://caniuse.com/mdn-css_selectors_user-invalid