[twic]

Firefox tells me:

  Web sites prove their identity via certificates. Firefox does not trust this site because it
  uses a certificate that is not valid for lingdong-.github.io. The certificate is only valid for
  the following names: www.github.com, *.github.com, github.com, *.github.io, github.io,
  *.githubusercontent.com, githubusercontent.com

  Error code: SSL_ERROR_BAD_CERT_DOMAIN

So it seems it thinks that *.github.io does not match lingdong-.github.io. Perhaps due to the trailing hyphen?