A minor change is needed to make safecss work properly when a site is being viewed under HTTPS context - otherwise it'll use "siteurl" directly from wp_options which, in all likelihood, is using the http:// protocol. This will cause mixed content errors and browsers will throw up scary looking "site not secure! omgwtfbbq" errors.
On line 433 of safecss/safecss.php, replace:
$href = get_option( 'siteurl' );
$href = site_url();