Merge branch 'v4' of https://github.com/twbs/derpstrap into v4

Conflicts:
	docs/components/helpers.md
	scss/_utilities.scss

scss/_buttons.scss

@@ -43,7 +43,7 @@
   }
 
   &.disabled,
-  &[disabled],
+  &:disabled,
   fieldset[disabled] & {
     pointer-events: none; // Future-proof disabling of clicks
     cursor: $cursor-disabled;
@@ -88,7 +88,7 @@
   &,
   &:active,
   &.active,
-  &[disabled],
+  &:disabled,
   fieldset[disabled] & {
     background-color: transparent;
     @include box-shadow(none);
@@ -105,7 +105,7 @@
     text-decoration: underline;
     background-color: transparent;
   }
-  &[disabled],
+  &:disabled,
   fieldset[disabled] & {
     &:hover,
     &:focus {

scss/_forms.scss

@@ -133,6 +133,7 @@ output {
   // Placeholder
   &::placeholder {
     color: $input-color-placeholder;
+    opacity: 1; // Override Firefox's unusual default opacity; see https://github.com/twbs/bootstrap/pull/11526
   }
 
   // Disabled and read-only inputs
@@ -140,7 +141,7 @@ output {
   // HTML5 says that controls under a fieldset > legend:first-child won't be
   // disabled if the fieldset is disabled. Due to implementation difficulty, we
   // don't honor that edge case; we style them as disabled anyway.
-  &[disabled],
+  &:disabled,
   &[readonly],
   fieldset[disabled] & {
     cursor: $cursor-disabled;
@@ -263,7 +264,7 @@ input[type="search"] {
 // Note: Neither radios nor checkboxes can be readonly.
 input[type="radio"],
 input[type="checkbox"] {
-  &[disabled],
+  &:disabled,
   &.disabled,
   fieldset[disabled] & {
     cursor: $cursor-disabled;

scss/_navbar.scss

@@ -309,7 +309,7 @@
     &:focus {
       color: $navbar-default-link-hover-color;
     }
-    &[disabled],
+    &:disabled,
     fieldset[disabled] & {
       &:hover,
       &:focus {
@@ -422,7 +422,7 @@
       color: $navbar-inverse-link-hover-color;
     }
 
-    &[disabled],
+    &:disabled,
     fieldset[disabled] & {
       &:hover,
       &:focus {

scss/_utilities.scss

@@ -46,12 +46,6 @@
   z-index: $zindex-navbar-fixed;
 }
 
-
-// Toggling content
-.show {
-  display: block !important;
-}
-
 // Always hide an element with the `hidden` HTML attribute (from PureCSS).
 [hidden] {
   display: none !important;
@@ -66,15 +60,6 @@
 }
 
 
-// Hide from screenreaders and browsers
-//
-// Credit: HTML5 Boilerplate
-.hidden {
-  display: none !important;
-  visibility: hidden !important;
-}
-
-
 // For Affix plugin
 .affix {
   position: fixed;

scss/mixins/_buttons.scss

@@ -28,7 +28,7 @@
     @include box-shadow(inset 0 3px 5px rgba(0,0,0,.125));
   }
   &.disabled,
-  &[disabled],
+  &:disabled,
   fieldset[disabled] & {
     &,
     &:hover,

scss/mixins/_image.scss

@@ -16,15 +16,13 @@
 
 // Retina image
 //
-// Short retina mixin for setting background-image and -size. Note that the
-// spelling of `min--moz-device-pixel-ratio` is intentional.
+// Short retina mixin for setting background-image and -size.
 
 @mixin img-retina($file-1x, $file-2x, $width-1x, $height-1x) {
   background-image: url("#{file-1x}");
 
   @media
   only screen and (-webkit-min-device-pixel-ratio: 2),
-  only screen and (   min--moz-device-pixel-ratio: 2),
   only screen and (     -o-min-device-pixel-ratio: 2/1),
   only screen and (        min-device-pixel-ratio: 2),
   only screen and (                min-resolution: 192dpi),