Apparence of invalid fields ?

In #496 I wanted to add this CSS rule:

```css
input:invalid, input:focus:invalid {
box-shadow: 0 0 5px 1px red;
}
```

And it made me realized that we had many places where forms are loaded with invalid values (usually empty strings :))


What should we do? Should we highlight them in red? Should we show them as red only when focused?

The issue names no file, test, or entry point. Start with the forms that load empty-string values and the CSS rule proposed in #496; done requires a decided treatment for invalid fields and verification across the affected forms.

Written by the indexing model from the issue text.