Documentation cleanup (inc. use of <input ... readonly> for static form controls example)

* Fix incorrect code indentation
* Remove unnecessary vendor prefix for `box-sizing` - all modern browsers now support this unprefixed
* Remove incorrect `<label>` and change static controls to readonly inputs
* Allow `<img>` elements without `src` to allow for `holder.js` images used in the docs, which lack `src` and use `data-src` instead

docs/.htmllintrc

@@ -9,6 +9,7 @@
   "html-valid-content-model": false,
   "id-class-ignore-regex": "(onclick|content|[a-z]+([A-Z][a-z])+)",
   "id-class-style": "dash",
+  "img-req-src": false,
   "img-req-alt": false,
   "indent-style": "spaces",
   "indent-width": 2,

docs/components/collapse.md

@@ -225,9 +225,7 @@ Bootstrap's collapse class exposes a few events for hooking into collapse functi
     </tr>
     <tr>
       <td>hide.bs.collapse</td>
-     <td>
-      This event is fired immediately when the <code>hide</code> method has been called.
-     </td>
+      <td>This event is fired immediately when the <code>hide</code> method has been called.</td>
     </tr>
     <tr>
       <td>hidden.bs.collapse</td>

docs/components/forms.md

@@ -376,7 +376,7 @@ Be sure to add `.col-form-label` to your `<label>`s as well so they're verticall
       </div>
     </fieldset>
     <div class="form-group row">
-      <label class="col-sm-2">Checkbox</label>
+      <div class="col-sm-2">Checkbox</div>
       <div class="col-sm-10">
         <div class="form-check">
           <label class="form-check-label">
@@ -569,14 +569,14 @@ Should you have no text within the `<label>`, the input is positioned as you'd e
 
 ## Static controls
 
-When you need to place plain text next to a form label within a form, use the `.form-control-static` class on an element of your choice.
+If you want to have read-only fields in your form styled as plain text, use the `.form-control-static` class to remove the default form field styling and preserve the correct margin and padding.
 
 {% example html %}
 <form>
   <div class="form-group row">
-    <label class="col-sm-2 col-form-label">Email</label>
+    <label for="staticEmail" class="col-sm-2 col-form-label">Email</label>
     <div class="col-sm-10">
-      <p class="form-control-static">email@example.com</p>
+      <input type="text" readonly class="form-control-static" id="staticEmail" value="email@example.com">
     </div>
   </div>
   <div class="form-group row">
@@ -591,8 +591,8 @@ When you need to place plain text next to a form label within a form, use the `.
 {% example html %}
 <form class="form-inline">
   <div class="form-group">
-    <label class="sr-only">Email</label>
-    <p class="form-control-static">email@example.com</p>
+    <label for="staticEmail2" class="sr-only">Email</label>
+    <input type="text" readonly class="form-control-static" id="staticEmail2" value="email@example.com">
   </div>
   <div class="form-group mx-sm-3">
     <label for="inputPassword2" class="sr-only">Password</label>
@@ -604,7 +604,7 @@ When you need to place plain text next to a form label within a form, use the `.
 
 ## Disabled states
 
-Add the `disabled` boolean attribute on an input to prevent user interactions. Disabled inputs appear lighter and add a `not-allowed` cursor.
+Add the `disabled` boolean attribute on an input to prevent user interactions and make it appear lighter.
 
 {% highlight html %}
 <input class="form-control" id="disabledInput" type="text" placeholder="Disabled input here..." disabled>

docs/getting-started/introduction.md

@@ -96,8 +96,6 @@ On the rare occasion you need to override it, use something like the following:
 
 {% highlight scss %}
 .selector-for-some-widget {
-  -webkit-box-sizing: content-box;
-     -moz-box-sizing: content-box;
   box-sizing: content-box;
 }
 {% endhighlight %}