$ cat /var/www/html/index.html
<!DOCTYPE html>
<html>
    <body>Hello, Validator!</body>
</html>
$ validatehtml /var/www/html/index.html
[ERROR] The character encoding was not declared. Proceeding using “windows-1252”.
[ERROR] Element “head” is missing a required instance of child element “title”.
From line 2, column 7; to line 3, column 10
ml> <html>     <body>Hello,
          ^^^^^^^^^^^
[WARNING] Consider adding a “lang” attribute to the “html” start tag to declare the language of this document.
From line 1, column 16; to line 2, column 6
TYPE html> <html>     <
          ^^^^^^^

Usage: validatehtml [-s URL] [-x DIR] FILE | DIR ...
  -s URL  --validator-url=URL  validation service url (default: https://validator.w3.org/nu/)
  -x DIR  --exclude=DIR        exclude files in DIR
  -1      --oneline            print each message on one line

Licensed under The 3-Clause BSD License.