External Checker not available

Is the HTML5 checking engine down or is something wrong on my end?


error:


The following notes and warnings highlight missing or conflicting
information which caused the validator to perform some guesswork prior to
validation, or other things affecting the output below. If the guess or
fallback is incorrect, it could make validation results entirely incoherent.
It is *highly recommended* to check these potential issues, and, if
necessary, fix them and re-validate the document.

   1.

   [image: Info] Using experimental feature: *HTML5 Conformance Checker*.

   The validator checked your document with an experimental feature: *HTML5
   Conformance Checker*. This feature has been made available for your
   convenience, but be aware that it may be unreliable, or not perfectly up to
   date with the latest development of some cutting-edge technologies. If you
   find any issues with this feature, please report them <feedback.html>.
   Thank you.


   1. [image: Error] 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)