Invalid styling only applied when placeholder is not shown

[juthilo]

We're not sure if this is the intended behavior: an input with label and placeholder that is marked as required and empty will never be styled as invalid because the invalid rule is restricted to :not(:placeholder-shown). We would argue that placeholders are intended to indicate what should be entered into a field and not that the field is optional.

[mfranzke]

You're right – it's a bad workaround until we get :user-invalid soon. At least the error message would still get displayed, but not the color for this constellation. I'll keep this ticket open so that we could use it as a reminder for :user-invalid.

[d-koppenhagen]

See #226