Have HTML5 validator ignore `autocomplete="off"` on input[type="checkbox"]

Not clear from the spec whether this is invalid per se.
But it is the current best known workaround for Firefox's unusual behavior.

Gruntfile.js

@@ -340,7 +340,8 @@ module.exports = function (grunt) {
         reset: true,
         relaxerror: [
           'Bad value X-UA-Compatible for attribute http-equiv on element meta.',
-          'Element img is missing required attribute src.'
+          'Element img is missing required attribute src.',
+          'Attribute autocomplete not allowed on element input at this point.'
         ]
       },
       files: {