Including a link to <http://jigsaw.w3.org/css-validator/check/referer> in a web
page will validate that page's CSS when the link is followed, based on the
refered URL provided by the user's web browser.

However, if access to the page is restricted using basic server authentication
(i.e. requires a user name and password), then the following error message is
received when following the above link on the page:

  'I/O Error: Basic realm="[realm of refered document]"'

The submitter is not prompted to enter any authentication details prior to
receiving the error message.

The validator can handle pages requiring authentication when submitted directly
using a URL of the form <http://jigsaw.w3.org/css-validator/validator?uri=[URL
of document to validate]> or manually via the web form interface -- in these
cases, the submitter is prompted to enter authentication details prior to the
page being validated, as expected.

Note that the markup validator can handle documents requiring authentication
that are submitted via a refered URL using the link
<http://validator.w3.org/check/referer>.