Merge pull request #15072 from twbs/html-validator-x-ua-compatible

HTML validation: No longer relax the `X-UA-Compatible` error

Gruntfile.js

@@ -321,7 +321,6 @@ module.exports = function (grunt) {
         failHard: true,
         reset: true,
         relaxerror: [
-          'Bad value X-UA-Compatible for attribute http-equiv on element meta.',
           'Element img is missing required attribute src.',
           'Attribute autocomplete not allowed on element input at this point.',
           'Attribute autocomplete not allowed on element button at this point.'