Fix broken links.

docs/_data/translations.yml

 - name: Chinese
   code: zh
   description: Bootstrap 中文文档
-  url: http://v4.bootcss.com/
+  url: https://v4.bootcss.com/
 
 - name: Chinese
   code: zh

docs/components/collapse.md

@@ -40,7 +40,7 @@ You can use a link with the `href` attribute, or a button with the `data-target`
 
 ## Accordion example
 
-Using the [card]({{ site.baseurl }}/components/card) component, you can extend the default collapse behavior to create an accordion.
+Using the [card]({{ site.baseurl }}/components/card/) component, you can extend the default collapse behavior to create an accordion.
 
 {% example html %}
 <div id="accordion" role="tablist">

docs/components/modal.md

@@ -347,7 +347,7 @@ Utilize the Bootstrap grid system within a modal by nesting `.container-fluid` w
 
 ### Varying modal content
 
-Have a bunch of buttons that all trigger the same modal with slightly different contents? Use `event.relatedTarget` and [HTML `data-*` attributes](https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Using_data_attributes) (possibly [via jQuery](https://api.jquery.com/data/)) to vary the contents of the modal depending on which button was clicked.
+Have a bunch of buttons that all trigger the same modal with slightly different contents? Use `event.relatedTarget` and [HTML `data-*` attributes](https://developer.mozilla.org/en-US/docs/Learn/HTML/Howto/Use_data_attributes) (possibly [via jQuery](https://api.jquery.com/data/)) to vary the contents of the modal depending on which button was clicked.
 
 Below is a live demo followed by example HTML and JavaScript. For more information, [read the modal events docs](#events) for details on `relatedTarget`.
 

docs/content/reboot.md

@@ -311,7 +311,7 @@ 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](http://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/getting-started/download.md

@@ -38,7 +38,7 @@ Pull in Bootstrap's **source files** into nearly any project with some of the mo
 
 ### npm
 
-Install Bootstrap in your Node powered apps with [the npm package](https://www.npmjs.org/package/bootstrap):
+Install Bootstrap in your Node powered apps with [the npm package](https://www.npmjs.com/package/bootstrap):
 
 {% highlight bash %}
 npm install bootstrap@{{ site.current_version }}

docs/layout/grid.md

@@ -585,7 +585,7 @@ To nest your content with the default grid, add a new `.row` and set of `.col-sm
 
 ## Sass mixins
 
-When using Bootstrap's source Sass files, you have the option of using Sass variables and mixins to create custom, semantic, and responsive page layouts. Our [predefined grid classes](#predefined-classes) use these same variables and mixins to provide a whole suite of ready-to-use classes for fast responsive layouts.
+When using Bootstrap's source Sass files, you have the option of using Sass variables and mixins to create custom, semantic, and responsive page layouts. Our predefined grid classes use these same variables and mixins to provide a whole suite of ready-to-use classes for fast responsive layouts.
 
 ### Variables
 

docs/utilities/responsive-helpers.md

@@ -14,7 +14,7 @@ Rules are directly applied to `<iframe>`, `<embed>`, `<video>`, and `<object>` e
 
 {% example html %}
 <div class="embed-responsive embed-responsive-16by9">
-  <iframe class="embed-responsive-item" src="//www.youtube.com/embed/zpOULjyy-n8?rel=0" allowfullscreen></iframe>
+  <iframe class="embed-responsive-item" src="https://www.youtube.com/embed/zpOULjyy-n8?rel=0" allowfullscreen></iframe>
 </div>
 {% endexample %}