Specify rel="noopener" for external links.

docs/_includes/css/less.html

@@ -161,7 +161,7 @@ a {
 
 
   <h3 id="less-mixins-box-sizing">Box-sizing</h3>
-  <p>Reset your components' box model with a single mixin. For context, see this <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/box-sizing" target="_blank">helpful article from Mozilla</a>.</p>
+  <p>Reset your components' box model with a single mixin. For context, see this <a href="https://developer.mozilla.org/en-US/docs/CSS/box-sizing" rel="noopener" target="_blank">helpful article from Mozilla</a>.</p>
   <p>The mixin is <strong>deprecated</strong> as of v3.2.0, with the introduction of Autoprefixer. To preserve backwards-compatibility, Bootstrap will continue to use the mixin internally until Bootstrap v4.</p>
 {% highlight scss %}
 .box-sizing(@box-model) {
@@ -390,7 +390,7 @@ a {
   <p>Utility mixins are mixins that combine otherwise unrelated CSS properties to achieve a specific goal or task.</p>
 
   <h3 id="less-mixins-clearfix">Clearfix</h3>
-  <p>Forget adding <code>class="clearfix"</code> to any element and instead add the <code>.clearfix()</code> mixin where appropriate. Uses the <a href="http://nicolasgallagher.com/micro-clearfix-hack/" target="_blank">micro clearfix</a> from <a href="https://twitter.com/necolas" target="_blank">Nicolas Gallagher</a>.</p>
+  <p>Forget adding <code>class="clearfix"</code> to any element and instead add the <code>.clearfix()</code> mixin where appropriate. Uses the <a href="http://nicolasgallagher.com/micro-clearfix-hack/" rel="noopener" target="_blank">micro clearfix</a> from <a href="https://twitter.com/necolas" rel="noopener" target="_blank">Nicolas Gallagher</a>.</p>
 {% highlight scss %}
 // Mixin
 .clearfix() {

docs/_includes/css/overview.html

@@ -33,7 +33,7 @@
   <p>These styles can be found within <code>scaffolding.less</code>.</p>
 
   <h2 id="overview-normalize">Normalize.css</h2>
-  <p>For improved cross-browser rendering, we use <a href="https://necolas.github.io/normalize.css/" target="_blank">Normalize.css</a>, a project by <a href="https://twitter.com/necolas" target="_blank">Nicolas Gallagher</a> and <a href="https://twitter.com/jon_neal" target="_blank">Jonathan Neal</a>.</p>
+  <p>For improved cross-browser rendering, we use <a href="https://necolas.github.io/normalize.css/" rel="noopener" target="_blank">Normalize.css</a>, a project by <a href="https://twitter.com/necolas" rel="noopener" target="_blank">Nicolas Gallagher</a> and <a href="https://twitter.com/jon_neal" rel="noopener" target="_blank">Jonathan Neal</a>.</p>
 
   <h2 id="overview-container">Containers</h2>
   <p>Bootstrap requires a containing element to wrap site contents and house our grid system. You may choose one of two containers to use in your projects. Note that, due to <code>padding</code> and more, neither container is nestable.</p>

docs/_includes/footer.html

@@ -10,9 +10,9 @@
       <li><a href="{{ site.baseurl }}/about/">About</a></li>
     </ul>
 
-    <p>Designed and built with all the love in the world by <a href="https://twitter.com/mdo" target="_blank">@mdo</a> and <a href="https://twitter.com/fat" target="_blank">@fat</a>. Maintained by the <a href="https://github.com/orgs/twbs/people">core team</a> with the help of <a href="https://github.com/twbs/bootstrap/graphs/contributors">our contributors</a>.</p>
+    <p>Designed and built with all the love in the world by <a href="https://twitter.com/mdo" rel="noopener" target="_blank">@mdo</a> and <a href="https://twitter.com/fat" rel="noopener" target="_blank">@fat</a>. Maintained by the <a href="https://github.com/orgs/twbs/people">core team</a> with the help of <a href="https://github.com/twbs/bootstrap/graphs/contributors">our contributors</a>.</p>
 
-    <p>Code licensed <a rel="license" href="https://github.com/twbs/bootstrap/blob/master/LICENSE" target="_blank">MIT</a>, docs <a rel="license" href="https://creativecommons.org/licenses/by/3.0/" target="_blank">CC BY 3.0</a>.</p>
+    <p>Code licensed <a href="https://github.com/twbs/bootstrap/blob/master/LICENSE" rel="license noopener" target="_blank">MIT</a>, docs <a href="https://creativecommons.org/licenses/by/3.0/" rel="license noopener" target="_blank">CC BY 3.0</a>.</p>
 
   </div>
 </footer>

docs/about.html

@@ -59,7 +59,7 @@ lead: "Learn about the project's history, meet the maintaining teams, and find o
 <div class="bs-docs-section">
   <h1 id="brand" class="page-header">Brand guidelines</h1>
 
-  <p class="lead">Have a need for Bootstrap's brand resources? Great! We have only a few guidelines we follow, and in turn ask you to follow as well. These guidelines were inspired by MailChimp's <a href="https://mailchimp.com/about/brand-assets/" target="_blank">Brand Assets</a>.</p>
+  <p class="lead">Have a need for Bootstrap's brand resources? Great! We have only a few guidelines we follow, and in turn ask you to follow as well. These guidelines were inspired by MailChimp's <a href="https://mailchimp.com/about/brand-assets/" rel="noopener" target="_blank">Brand Assets</a>.</p>
 
   <h2>Mark and logo</h2>
   <p>Use either the Bootstrap mark (a capital <strong>B</strong>) or the standard logo (just <strong>Bootstrap</strong>). It should always appear in Helvetica Neue Bold. <strong>Do not use the Twitter bird</strong> in association with Bootstrap.</p>

docs/customize.html

@@ -357,7 +357,7 @@ lead: Customize Bootstrap's components, Less variables, and jQuery plugins to ge
 
     <div class="bs-callout bs-callout-danger">
       <h4>jQuery required</h4>
-      <p>All plugins require the latest version of <a href="https://jquery.com/" target="_blank">jQuery</a> to be included.</p>
+      <p>All plugins require the latest version of <a href="https://jquery.com/" rel="noopener" target="_blank">jQuery</a> to be included.</p>
     </div>
   </div>
 

docs/index.html

@@ -74,7 +74,7 @@ title: Bootstrap · The world's most popular mobile-first and responsive f
     <div class="row bs-docs-featured-sites">
     {% for showcase in site.data.showcase %}
       <div class="col-xs-8 col-xs-offset-2 col-sm-4 col-sm-offset-0">
-        <a href="{{ showcase.expo_url }}" target="_blank" title="{{ showcase.name }}">
+        <a href="{{ showcase.expo_url }}" rel="noopener" target="_blank" title="{{ showcase.name }}">
           <img src="{{ site.baseurl }}/assets/img/expo-{{ showcase.img }}.jpg" alt="{{ showcase.name }}" class="img-responsive" width="800" height="600">
         </a>
       </div>