sigh. That used to work. The book has been updated to say:
By adding a bit of css, we can also highlight the fields with errors in red. Place
the following code in app/assets/stylesheets/field_with_errors.css:
.field_with_errors :is(input, textarea, select) {
border: solid red 2px !important;
}
If we try again, the result is more impressive.