http://validator.w3.org/#validate_by_input External Checker not available 500 Can't connect to localhost:8888 (connect: Connection refused)

1.	 External Checker not available 

Checking the Document Type of this document requires the help of an
external tool which was either not enabled in this validator, or is
currently unavailable. Check in the validator's system configuration
that HTML5 Validator is enabled and functional. 

The error encountered was: 500 Can't connect to localhost:8888 (connect:
Connection refused) 

 

NOTE: Whenever possible, give the address of the document you were
checking.