Normalize all links. (#24109)

* use a trailing slash when possible
* use https when possible
* remove a few redirected links
* consistently use `https://popper.js.org/`
* fix `iconUrl` in nuget files
* change Jekyll Windows guide to the official one

docs/4.0/components/card.md

@@ -560,7 +560,7 @@ Just like with card groups, card footers in decks will automatically line up.
 
 ### Card columns
 
-Cards can be organized into [Masonry](http://masonry.desandro.com)-like columns with just CSS by wrapping them in `.card-columns`. Cards are built with CSS `column` properties instead of flexbox for easier alignment. Cards are ordered from top to bottom and left to right.
+Cards can be organized into [Masonry](https://masonry.desandro.com/)-like columns with just CSS by wrapping them in `.card-columns`. Cards are built with CSS `column` properties instead of flexbox for easier alignment. Cards are ordered from top to bottom and left to right.
 
 **Heads up!** Your mileage with card columns may vary. To prevent cards breaking across columns, we must set them to `display: inline-block` as `column-break-inside: avoid` isn't a bulletproof solution yet.
 

docs/4.0/components/dropdowns.md

@@ -10,7 +10,7 @@ toc: true
 
 Dropdowns are toggleable, contextual overlays for displaying lists of links and more. They're made interactive with the included Bootstrap dropdown JavaScript plugin. They're toggled by clicking, not by hovering; this is [an intentional design decision.](http://markdotto.com/2012/02/27/bootstrap-explained-dropdowns/)
 
-Dropdowns are built on a third party library, [Popper.js](https://popper.js.org), which provides dynamic positioning and viewport detection. Be sure to include [popper.min.js]({{ site.cdn.popper }}) before Bootstrap's JavaScript or use `bootstrap.bundle.min.js` / `bootstrap.bundle.js` which contains Popper.js.
+Dropdowns are built on a third party library, [Popper.js](https://popper.js.org/), which provides dynamic positioning and viewport detection. Be sure to include [popper.min.js]({{ site.cdn.popper }}) before Bootstrap's JavaScript or use `bootstrap.bundle.min.js` / `bootstrap.bundle.js` which contains Popper.js.
 
 If you're building our JS from source, it [requires `util.js`]({{ site.baseurl }}/docs/{{ site.docs_version }}/getting-started/javascript/#util).
 

docs/4.0/components/forms.md

@@ -672,7 +672,7 @@ While Bootstrap will apply these styles in all browsers, Internet Explorer 11 an
 
 ## Validation
 
-Provide valuable, actionable feedback to your users with HTML5 form validation–[available in all our supported browsers](http://caniuse.com/#feat=form-validation). Choose from the browser default validation feedback, or implement custom messages with our built-in classes and starter JavaScript.
+Provide valuable, actionable feedback to your users with HTML5 form validation–[available in all our supported browsers](https://caniuse.com/#feat=form-validation). Choose from the browser default validation feedback, or implement custom messages with our built-in classes and starter JavaScript.
 
 {% callout warning %}
 We **highly recommend** custom validation styles as native browser defaults are not announced to screen readers.

docs/4.0/components/navbar.md

@@ -395,7 +395,7 @@ When the container is within your navbar, its horizontal padding is removed at b
 
 ## Placement
 
-Use our position utilities to place navbars in non-static positions. Choose from fixed to the top, fixed to the bottom, or stickied to the top. **Note that `position: sticky`, used for `.sticky-top`, [isn't fully supported in every browser](http://caniuse.com/#feat=css-sticky).**
+Use our position utilities to place navbars in non-static positions. Choose from fixed to the top, fixed to the bottom, or stickied to the top. **Note that `position: sticky`, used for `.sticky-top`, [isn't fully supported in every browser](https://caniuse.com/#feat=css-sticky).**
 
 {% example html %}
 <nav class="navbar navbar-light bg-light">

docs/4.0/components/popovers.md

@@ -11,7 +11,7 @@ toc: true
 Things to know when using the popover plugin:
 
 
-- Popovers rely on the 3rd party library [Popper.js](https://popper.js.org) for positioning. You must include [popper.min.js]({{ site.cdn.popper }}) before bootstrap.js or use `bootstrap.bundle.min.js` / `bootstrap.bundle.js` which contains Popper.js in order for popovers to work!
+- Popovers rely on the 3rd party library [Popper.js](https://popper.js.org/) for positioning. You must include [popper.min.js]({{ site.cdn.popper }}) before bootstrap.js or use `bootstrap.bundle.min.js` / `bootstrap.bundle.js` which contains Popper.js in order for popovers to work!
 - Popovers require the [tooltip plugin]({{ site.baseurl }}/docs/{{ site.docs_version }}/components/tooltips/) as a dependency.
 - If building our JS from source, it [requires `util.js`]({{ site.baseurl }}/docs/{{ site.docs_version }}/getting-started/javascript/#util).
 - Popovers are opt-in for performance reasons, so **you must initialize them yourself**.

docs/4.0/components/tooltips.md

@@ -10,7 +10,7 @@ toc: true
 
 Things to know when using the tooltip plugin:
 
-- Tooltips rely on the 3rd party library [Popper.js](https://popper.js.org) for positioning. You must include [popper.min.js]({{ site.cdn.popper }}) before bootstrap.js or use `bootstrap.bundle.min.js` / `bootstrap.bundle.js` which contains Popper.js in order for tooltips to work!
+- Tooltips rely on the 3rd party library [Popper.js](https://popper.js.org/) for positioning. You must include [popper.min.js]({{ site.cdn.popper }}) before bootstrap.js or use `bootstrap.bundle.min.js` / `bootstrap.bundle.js` which contains Popper.js in order for tooltips to work!
 - If building our JS from source, it [requires `util.js`]({{ site.baseurl }}/docs/{{ site.docs_version }}/getting-started/javascript/#util).
 - Tooltips are opt-in for performance reasons, so **you must initialize them yourself**.
 - Tooltips with zero-length titles are never displayed.

docs/4.0/content/reboot.md

@@ -330,7 +330,7 @@ The `<abbr>` element receives basic styling to make it stand out amongst paragra
 
 ## HTML5 `[hidden]` attribute
 
-HTML5 adds [a new global attribute named `[hidden]`](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/hidden), which is styled as `display: none` by default. Borrowing an idea from [PureCSS](https://purecss.io), we improve upon this default by making `[hidden] { display: none !important; }` to help prevent its `display` from getting accidentally overridden. While `[hidden]` isn't natively supported by IE10, the explicit declaration in our CSS gets around that problem.
+HTML5 adds [a new global attribute named `[hidden]`](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/hidden), which is styled as `display: none` by default. Borrowing an idea from [PureCSS](https://purecss.io/), we improve upon this default by making `[hidden] { display: none !important; }` to help prevent its `display` from getting accidentally overridden. While `[hidden]` isn't natively supported by IE10, the explicit declaration in our CSS gets around that problem.
 
 {% highlight html %}
 <input type="text" hidden>

docs/4.0/examples/blog/index.html

@@ -148,7 +148,7 @@
     </main><!-- /.container -->
 
     <footer class="blog-footer">
-      <p>Blog template built for <a href="https://getbootstrap.com">Bootstrap</a> by <a href="https://twitter.com/mdo">@mdo</a>.</p>
+      <p>Blog template built for <a href="https://getbootstrap.com/">Bootstrap</a> by <a href="https://twitter.com/mdo">@mdo</a>.</p>
       <p>
         <a href="#">Back to top</a>
       </p>

docs/4.0/examples/cover/index.html

@@ -45,7 +45,7 @@
 
           <footer class="mastfoot">
             <div class="inner">
-              <p>Cover template for <a href="https://getbootstrap.com">Bootstrap</a>, by <a href="https://twitter.com/mdo">@mdo</a>.</p>
+              <p>Cover template for <a href="https://getbootstrap.com/">Bootstrap</a>, by <a href="https://twitter.com/mdo">@mdo</a>.</p>
             </div>
           </footer>
 

docs/4.0/getting-started/browsers-devices.md

@@ -101,7 +101,7 @@ For a list of some of the browser bugs that Bootstrap has to grapple with, see o
 
 ## Internet Explorer
 
-Internet Explorer 10+ is supported; IE9 and down is not. Please be aware that some CSS3 properties and HTML5 elements are not fully supported in IE10, or require prefixed properties for full functionality. Visit [Can I use...](http://caniuse.com/) for details on browser support of CSS3 and HTML5 features.
+Internet Explorer 10+ is supported; IE9 and down is not. Please be aware that some CSS3 properties and HTML5 elements are not fully supported in IE10, or require prefixed properties for full functionality. Visit [Can I use...](https://caniuse.com/) for details on browser support of CSS3 and HTML5 features.
 
 **If you require IE8-9 support, use Bootstrap 3.** It's the most stable version of our code and is still supported by our team for critical bugfixes and documentation changes. However, no new features will be added to it.
 
@@ -167,7 +167,7 @@ Want to see an example? [Check out this JS Bin demo.](http://jsbin.com/OyaqoDO/2
 
 ## Validators
 
-In order to provide the best possible experience to old and buggy browsers, Bootstrap uses [CSS browser hacks](http://browserhacks.com) in several places to target special CSS to certain browser versions in order to work around bugs in the browsers themselves. These hacks understandably cause CSS validators to complain that they are invalid. In a couple places, we also use bleeding-edge CSS features that aren't yet fully standardized, but these are used purely for progressive enhancement.
+In order to provide the best possible experience to old and buggy browsers, Bootstrap uses [CSS browser hacks](http://browserhacks.com/) in several places to target special CSS to certain browser versions in order to work around bugs in the browsers themselves. These hacks understandably cause CSS validators to complain that they are invalid. In a couple places, we also use bleeding-edge CSS features that aren't yet fully standardized, but these are used purely for progressive enhancement.
 
 These validation warnings don't matter in practice since the non-hacky portion of our CSS does fully validate and the hacky portions don't interfere with the proper functioning of the non-hacky portion, hence why we deliberately ignore these particular warnings.
 

docs/4.0/getting-started/build-tools.md

@@ -15,7 +15,7 @@ To use our build system and run our documentation locally, you'll need a copy of
 1. [Download and install Node](https://nodejs.org/download/), which we use to manage our dependencies.
 2. Navigate to the root `/bootstrap` directory and run `npm install` to install our local dependencies listed in [package.json]({{ site.repo }}/blob/v{{ site.current_version }}/package.json).
 3. [Install Ruby][install-ruby], install [Bundler][gembundler] with `gem install bundler`, and finally run `bundle install`. This will install all Ruby dependencies, such as Jekyll and plugins.
-  - **Windows users:** Read [this unofficial guide](http://jekyll-windows.juthilo.com/) to get Jekyll up and running without problems.
+  - **Windows users:** Read [this guide](https://jekyllrb.com/docs/windows/) to get Jekyll up and running without problems.
 
 When completed, you'll be able to run the various commands provided from the command line.
 

docs/4.0/getting-started/download.md

@@ -81,7 +81,7 @@ gem install bootstrap -v 4.0.0.beta
 
 ### Composer
 
-You can also install and manage Bootstrap's Sass and JavaScript using [Composer](https://getcomposer.org):
+You can also install and manage Bootstrap's Sass and JavaScript using [Composer](https://getcomposer.org/):
 
 {% highlight sh %}
 composer require twbs/bootstrap:{{ site.current_version }}
@@ -89,7 +89,7 @@ composer require twbs/bootstrap:{{ site.current_version }}
 
 ### NuGet
 
-If you develop in .NET, you can also install and manage Bootstrap's [CSS](https://www.nuget.org/packages/bootstrap/) or [Sass](https://www.nuget.org/packages/bootstrap.sass/) and JavaScript using [NuGet](https://www.nuget.org):
+If you develop in .NET, you can also install and manage Bootstrap's [CSS](https://www.nuget.org/packages/bootstrap/) or [Sass](https://www.nuget.org/packages/bootstrap.sass/) and JavaScript using [NuGet](https://www.nuget.org/):
 
 {% highlight powershell %}
 Install-Package bootstrap -Pre

docs/4.0/getting-started/introduction.md

@@ -20,7 +20,7 @@ Copy-paste the stylesheet `<link>` into your `<head>` before all other styleshee
 <link rel="stylesheet" href="{{ site.cdn.css }}" integrity="{{ site.cdn.css_hash }}" crossorigin="anonymous">
 {% endhighlight %}
 
-Add our optional JavaScript plugins, jQuery, and [Popper.js](https://github.com/FezVrasta/popper.js) near the end of your pages, right before the closing `</body>` tag. Be sure to place jQuery and Popper.js first, as our JavaScript plugins depend on them. While we use [jQuery's slim build](https://blog.jquery.com/2016/06/09/jquery-3-0-final-released/) in our docs, the full version is also supported.
+Add our optional JavaScript plugins, jQuery, and [Popper.js](https://popper.js.org/) near the end of your pages, right before the closing `</body>` tag. Be sure to place jQuery and Popper.js first, as our JavaScript plugins depend on them. While we use [jQuery's slim build](https://blog.jquery.com/2016/06/09/jquery-3-0-final-released/) in our docs, the full version is also supported.
 
 {% highlight html %}
 <script src="{{ site.cdn.jquery }}" integrity="{{ site.cdn.jquery_hash }}" crossorigin="anonymous"></script>

docs/4.0/getting-started/javascript.md

@@ -14,7 +14,7 @@ Plugins can be included individually (using Bootstrap's individual `*.js` files)
 
 Some plugins and CSS components depend on other plugins. If you include plugins individually, make sure to check for these dependencies in the docs. Also note that **all plugins depend on jQuery** (this means jQuery must be included **before** the plugin files). [Consult our `package.json`]({{ site.repo }}/blob/v{{ site.current_version }}/package.json) to see which versions of jQuery are supported.
 
-Our tooltips and popovers also depend on [Popper.js](https://github.com/FezVrasta/popper.js).
+Our tooltips and popovers also depend on [Popper.js](https://popper.js.org/).
 
 ## Data attributes
 

docs/4.0/migration.md

@@ -254,7 +254,7 @@ Dropped entirely for the new card component.
 
 ### Vendor prefix mixins
 
-Bootstrap 3's [vendor prefix](http://webdesign.about.com/od/css/a/css-vendor-prefixes.htm) mixins, which were deprecated in v3.2.0, have been removed in Bootstrap 4. Since we use [Autoprefixer](https://github.com/postcss/autoprefixer), they're no longer necessary.
+Bootstrap 3's [vendor prefix](https://www.thoughtco.com/css-vendor-prefixes-3466867) mixins, which were deprecated in v3.2.0, have been removed in Bootstrap 4. Since we use [Autoprefixer](https://github.com/postcss/autoprefixer), they're no longer necessary.
 
 Removed the following mixins: `animation`, `animation-delay`, `animation-direction`, `animation-duration`, `animation-fill-mode`, `animation-iteration-count`, `animation-name`, `animation-timing-function`, `backface-visibility`, `box-sizing`, `content-columns`, `hyphens`, `opacity`, `perspective`, `perspective-origin`, `rotate`, `rotateX`, `rotateY`, `scale`, `scaleX`, `scaleY`, `skew`, `transform-origin`, `transition-delay`, `transition-duration`, `transition-property`, `transition-timing-function`, `transition-transform`, `translate`, `translate3d`, `user-select`