Add HTMLHint to the build; fixes #20297 (#20301)

[skip sauce]

Gruntfile.js

@@ -278,6 +278,9 @@ module.exports = function (grunt) {
       'postcss-docs': {
         command: 'npm run postcss-docs'
       },
+      htmlhint: {
+        command: 'npm run htmlhint'
+      },
       'upload-preview': {
         command: './grunt/upload-preview.sh'
       }
@@ -327,7 +330,7 @@ module.exports = function (grunt) {
   require('time-grunt')(grunt);
 
   // Docs HTML validation task
-  grunt.registerTask('validate-html', ['jekyll:docs', 'htmllint']);
+  grunt.registerTask('validate-html', ['jekyll:docs', 'htmllint', 'exec:htmlhint']);
 
   var runSubset = function (subset) {
     return !process.env.TWBS_TEST || process.env.TWBS_TEST === subset;

docs/.htmlhintrc

+{
+  "attr-lowercase": true,
+  "attr-no-duplication": true,
+  "alt-require": true,
+  "attr-unsafe-chars": true,
+  "attr-value-double-quotes": true,
+  "doctype-first": true,
+  "doctype-html5": true,
+  "id-unique": true,
+  "spec-char-escape": true,
+  "src-not-empty": true,
+  "tag-pair": true,
+  "tagname-lowercase": true,
+  "title-require": true
+}

package.json

@@ -20,6 +20,7 @@
     "change-version": "node grunt/change-version.js",
     "eslint": "eslint --config js/.eslintrc.json js/src",
     "jscs": "jscs --config=js/.jscsrc js/src js/tests/unit docs/assets/js/src grunt Gruntfile.js docs/assets/js/ie-emulation-modes-warning.js docs/assets/js/ie10-viewport-bug-workaround.js",
+    "htmlhint": "htmlhint --config docs/.htmlhintrc _gh_pages/",
     "postcss": "postcss --config grunt/postcss.js --replace dist/css/*.css",
     "postcss-docs": "postcss --config grunt/postcss.js --no-map --replace docs/assets/css/docs.min.css && postcss --config grunt/postcss.js --no-map --replace docs/examples/**/*.css",
     "shrinkwrap": "npm shrinkwrap --dev && shx mv ./npm-shrinkwrap.json ./grunt/npm-shrinkwrap.json",
@@ -64,6 +65,7 @@
     "grunt-saucelabs": "^9.0.0",
     "grunt-scss-lint": "^0.3.8",
     "grunt-stamp": "^0.3.0",
+    "htmlhint": "^0.9.13",
     "is-travis": "^1.0.0",
     "jscs": "^3.0.4",
     "load-grunt-tasks": "^3.4.0",