External link https://wiki.apache.org/httpd/RewriteHTTPToHTTPS failed: response code 0 means something's wrong.
It's possible libcurl couldn't connect to the server or perhaps the request timed out.
Sometimes, making too many requests at once also breaks things.
Either way, the return message (if any) from the server is: SSL connect error
htmlproofer 3.7.6 | Error: HTML-Proofer found 1 failure!