self-signed cert

While using the W3C Markup Validation Service, I got an error, but not
for any html. For connecting.

The error I got was:

 500 Can't connect to y.st:443

As you all can see, the website is https://y.st/ which uses a
self-signed certificate. There is nothing at http://y.st/ but a redirect
to https.

I found an example invalid SSL website:

https://www.pcwebshop.co.uk/

Which generates the same error:

 500 Can't connect to www.pcwebshop.co.uk:443

https://validator.w3.org/check?uri=https%3A%2F%2Fwww.pcwebshop.co.uk

It seems that validator.w3.org is lacking support for invalid ssl
certificates, however it could be some other error with y.st.