Upon validation error, show red border around Hierarchical Select

1) It's not a square but an asterisk.
2) You can simply enable the "Required" checkbox. That should work.

I just tested it, and an error appears if I don't select anything.

Ah … you were talking about the border. Then this is indeed a feature request :)

Should be doable.

It is far more complex than just some CSS. From a report I'm writing on the vast array of deficiencies of the Forms API:

- The 'error' class, set by form_error(), should be applied automatically to
all child form items of a form item.
- It is in fact impossible to set the 'error' class for child form items: it
must be set in the validation stage (#element_validate), but changes to the
form item at this point are not stored (the form and thus the form items are
passed by value, not by reference). One might expect to be able to use
form_set_error() with the empty string (it is even the default), but due to
a bug in _form_set_class(), that is also impossible:
if (form_get_error($element)) {
$class[] = 'error';
}
Since the empty string cast to a boolean results in FALSE, the 'error' class
is never added. The only way around this is by setting your custom #theme
function for the form item, allow for the empty string to result in the
'error' class being added by calling _form_set_class() again there.

Despite all of that, I've been able to write a work-around. Patch attached. Let me know if it works for you too.

P.S.: this has cost me about 2 hours. Way more than expected.

Committed.

- D6: http://drupal.org/cvs?commit=265606
- D5: http://drupal.org/cvs?commit=265608

Yep, I just tested it again, and it works.

Committed means it is in CVS (http://cvs.drupal.org). It's possible that it's not yet in the .tar.gz file (aka tarball), which is regenerated automatically on a daily basis, *not* after every change.

Thanks for your confirmation! :)