Between 2020-05-16 and 2020-05-18 our Crossmark widget was not visible to most users, and for users who did access the dialog via PDF, the dialog would have displayed without styling or images, or shown a security error.
The TLS certificate that we used for crossmark-cdn.crossref.org and assets.crossref.org expired. We fixed the certificate on 2020-05-18. We took action to ensure this won't happen again by using auto-renewing certificates. We also reviewed all of our content delivery network configuration to confirm that all certificates are auto-renewing.