Merge branch 'v4-dev' into v4-navbars

44 files changed, 513 insertions, 374 deletions

diff --git a/scss/.scss-lint.yml b/scss/.scss-lint.yml
index af837e4d0..9c6ac66ab 100644
--- a/scss/.scss-lint.yml
+++ b/scss/.scss-lint.yml
@@ -3,6 +3,9 @@ scss_files:
   - "**/*.scss"
   - "docs/assets/scss/**/*.scss"
 
+exclude:
+  - "scss/_normalize.scss"
+
 plugin_directories: ['.scss-linters']
 
 # List of gem names to load custom linters from (make sure they are already
@@ -89,7 +92,7 @@ linters:
     enabled: true
 
   ImportantRule:
-    enabled: true
+    enabled: false
 
   ImportPath:
     enabled: true
@@ -457,7 +460,7 @@ linters:
 
   Shorthand:
     enabled: true
-    allowed_shorthands: [1, 2, 3]
+    allowed_shorthands: [1, 2, 3, 4]
 
   SingleLinePerProperty:
     enabled: false
diff --git a/scss/_alert.scss b/scss/_alert.scss
index 55fc101dc..433177101 100644
--- a/scss/_alert.scss
+++ b/scss/_alert.scss
@@ -26,13 +26,13 @@
 // Expand the right padding and account for the close button's positioning.
 
 .alert-dismissible {
-  padding-right: ($alert-padding + 20px);
+  padding-right: ($alert-padding * 2);
 
   // Adjust close link position
   .close {
     position: relative;
-    top: -2px;
-    right: -21px;
+    top: -.125rem;
+    right: -$alert-padding;
     color: inherit;
   }
 }
diff --git a/scss/_animation.scss b/scss/_animation.scss
index edd7147e5..f0dfff81f 100644
--- a/scss/_animation.scss
+++ b/scss/_animation.scss
@@ -9,12 +9,21 @@
 
 .collapse {
   display: none;
-
   &.in {
     display: block;
   }
-  // tr&.in    { display: table-row; }
-  // tbody&.in { display: table-row-group; }
+}
+
+tr {
+  &.collapse.in {
+    display: table-row;
+  }
+}
+
+tbody {
+  &.collapse.in {
+    display: table-row-group;
+  }
 }
 
 .collapsing {
diff --git a/scss/_button-group.scss b/scss/_button-group.scss
index 60e9f7a74..0e63ecc54 100644
--- a/scss/_button-group.scss
+++ b/scss/_button-group.scss
@@ -10,6 +10,7 @@
   > .btn {
     position: relative;
     float: left;
+    margin-bottom: 0;
 
     // Bring the "active" button to the front
     &:focus,
diff --git a/scss/_card.scss b/scss/_card.scss
index a419d4d08..01a5733d6 100644
--- a/scss/_card.scss
+++ b/scss/_card.scss
@@ -70,6 +70,7 @@
 .card-header {
   @include clearfix;
   padding: $card-spacer-y $card-spacer-x;
+  margin-bottom: 0; // Removes the default margin-bottom of <hN>
   background-color: $card-cap-bg;
   border-bottom: $card-border-width solid $card-border-color;
 
@@ -168,7 +169,7 @@
 // Card image
 .card-img {
   // margin: -1.325rem;
-  @include border-radius(.25rem);
+  @include border-radius($card-border-radius-inner);
 }
 .card-img-overlay {
   position: absolute;
@@ -314,7 +315,7 @@
     column-gap: $card-columns-sm-up-column-gap;
 
     .card {
-      display: inline-block;
+      display: inline-block; // Don't let them vertically span multiple columns
       width: 100%; // Don't let them exceed the column width
     }
   }
diff --git a/scss/_custom-forms.scss b/scss/_custom-forms.scss
index a5f0bdfc4..e74f5c744 100644
--- a/scss/_custom-forms.scss
+++ b/scss/_custom-forms.scss
@@ -2,7 +2,7 @@
 
 // Embedded icons from Open Iconic.
 // Released under MIT and copyright 2014 Waybury.
-// http://useiconic.com/open
+// https://useiconic.com/open
 
 
 // Checkboxes and radios
@@ -11,7 +11,7 @@
 
 .custom-control {
   position: relative;
-  display: inline;
+  display: inline-block;
   padding-left: $custom-control-gutter;
   cursor: pointer;
 
@@ -61,7 +61,7 @@
 
 .custom-control-indicator {
   position: absolute;
-  top: .0625rem;
+  top: .25rem;
   left: 0;
   display: block;
   width: $custom-control-indicator-size;
@@ -117,13 +117,8 @@
 
 .custom-controls-stacked {
   .custom-control {
-    display: inline;
-
-    &::after {
-      display: block;
-      margin-bottom: $custom-control-spacer-y;
-      content: "";
-    }
+    float: left;
+    clear: left;
 
     + .custom-control {
       margin-left: 0;
@@ -142,6 +137,8 @@
 .custom-select {
   display: inline-block;
   max-width: 100%;
+  $select-border-width: ($border-width * 2);
+  height: calc(#{$input-height} - #{$select-border-width});
   padding: $custom-select-padding-y ($custom-select-padding-x + $custom-select-indicator-padding) $custom-select-padding-y $custom-select-padding-x;
   padding-right: $custom-select-padding-x \9;
   color: $custom-select-color;
diff --git a/scss/_dropdown.scss b/scss/_dropdown.scss
index e696d28be..47702e795 100644
--- a/scss/_dropdown.scss
+++ b/scss/_dropdown.scss
@@ -141,6 +141,7 @@
 .dropdown-header {
   display: block;
   padding: $dropdown-padding-y $dropdown-item-padding-x;
+  margin-bottom: 0; // for use with heading elements
   font-size: $font-size-sm;
   color: $dropdown-header-color;
   white-space: nowrap; // as with > li > a
diff --git a/scss/_forms.scss b/scss/_forms.scss
index 263a8bc15..be19a0795 100644
--- a/scss/_forms.scss
+++ b/scss/_forms.scss
@@ -18,8 +18,16 @@
   background-image: none;
   background-clip: padding-box;
   border: $input-btn-border-width solid $input-border-color;
+
   // Note: This has no effect on <select>s in some browsers, due to the limited stylability of `<select>`s in CSS.
-  @include border-radius($input-border-radius);
+  @if $enable-rounded {
+    // Manually use the if/else instead of the mixin to account for iOS override
+    border-radius: $input-border-radius;
+  } @else {
+    // Otherwise undo the iOS default
+    border-radius: 0;
+  }
+
   @include box-shadow($input-box-shadow);
   @include transition(border-color ease-in-out .15s, box-shadow ease-in-out .15s);
 
@@ -58,7 +66,8 @@
 
 select.form-control {
   &:not([size]):not([multiple]) {
-    height: $input-height;
+    $select-border-width: ($border-width * 2);
+    height: calc(#{$input-height} - #{$select-border-width});
   }
 
   &:focus::-ms-value {
@@ -124,12 +133,11 @@ select.form-control {
 // horizontal form layout.
 
 .form-control-static {
-  min-height: $input-height;
-  // Size it appropriately next to real form controls
   padding-top: $input-padding-y;
   padding-bottom: $input-padding-y;
-  // Remove default margin from `p`
-  margin-bottom: 0;
+  line-height: $input-line-height;
+  border: solid transparent;
+  border-width: 1px 0;
 
   &.form-control-sm,
   &.form-control-lg {
@@ -239,6 +247,7 @@ select.form-control-lg {
   }
 
   &.disabled {
+    color: $text-muted;
     cursor: $cursor-disabled;
   }
 }
@@ -321,6 +330,7 @@ select.form-control-lg {
 
     .input-group {
       display: inline-table;
+      width: auto;
       vertical-align: middle;
 
       .input-group-addon,
diff --git a/scss/_images.scss b/scss/_images.scss
index 989426999..3cdedc4ff 100644
--- a/scss/_images.scss
+++ b/scss/_images.scss
@@ -6,13 +6,9 @@
 // which weren't expecting the images within themselves to be involuntarily resized.
 // See also https://github.com/twbs/bootstrap/issues/18178
 .img-fluid {
-  @include img-fluid();
+  @include img-fluid;
 }
 
-// Rounded corners
-.img-rounded {
-  @include border-radius($border-radius-lg);
-}
 
 // Image thumbnails
 .img-thumbnail {
@@ -24,12 +20,7 @@
   @include box-shadow($thumbnail-box-shadow);
 
   // Keep them at most 100% wide
-  @include img-fluid(inline-block);
-}
-
-// Perfect circle
-.img-circle {
-  border-radius: 50%;
+  @include img-fluid;
 }
 
 //
diff --git a/scss/_input-group.scss b/scss/_input-group.scss
index fff9cb560..b447a3282 100644
--- a/scss/_input-group.scss
+++ b/scss/_input-group.scss
@@ -90,18 +90,18 @@
   text-align: center;
   background-color: $input-group-addon-bg;
   border: $input-btn-border-width solid $input-group-addon-border-color;
-  @include border-radius($border-radius);
+  @include border-radius($input-border-radius);
 
   // Sizing
   &.form-control-sm {
     padding: $input-padding-y-sm $input-padding-x-sm;
     font-size: $font-size-sm;
-    @include border-radius($border-radius-sm);
+    @include border-radius($input-border-radius-sm);
   }
   &.form-control-lg {
     padding: $input-padding-y-lg $input-padding-x-lg;
     font-size: $font-size-lg;
-    @include border-radius($border-radius-lg);
+    @include border-radius($input-border-radius-lg);
   }
 
   // scss-lint:disable QualifyingElement
diff --git a/scss/_list-group.scss b/scss/_list-group.scss
index d7e2ec5c1..1e0d1121d 100644
--- a/scss/_list-group.scss
+++ b/scss/_list-group.scss
@@ -70,6 +70,8 @@
 
 .list-group-flush {
   .list-group-item {
+    border-right: 0;
+    border-left: 0;
     border-radius: 0;
   }
 }
diff --git a/scss/_media.scss b/scss/_media.scss
index d970c946a..d1ebbcdeb 100644
--- a/scss/_media.scss
+++ b/scss/_media.scss
@@ -1,7 +1,6 @@
 @if $enable-flex {
   .media {
     display: flex;
-    margin-bottom: $spacer;
   }
   .media-body {
     flex: 1;
@@ -13,13 +12,6 @@
     align-self: flex-end;
   }
 } @else {
-  .media {
-    margin-top: $media-margin-top;
-
-    &:first-child {
-      margin-top: 0;
-    }
-  }
   .media,
   .media-body {
     overflow: hidden;
diff --git a/scss/_mixins.scss b/scss/_mixins.scss
index 1b5bf0fae..e53ab3414 100644
--- a/scss/_mixins.scss
+++ b/scss/_mixins.scss
@@ -51,4 +51,4 @@
 // @import "mixins/navbar-align";
 @import "mixins/grid-framework";
 @import "mixins/grid";
-@import "mixins/pulls";
+@import "mixins/float";
diff --git a/scss/_modal.scss b/scss/_modal.scss
index 83a133531..c326d5699 100644
--- a/scss/_modal.scss
+++ b/scss/_modal.scss
@@ -1,7 +1,7 @@
 // .modal-open      - body class for killing the scroll
 // .modal           - container to scroll within
 // .modal-dialog    - positioning shell for the actual modal
-// .modal-content   - actual modal w/ bg and corners and shit
+// .modal-content   - actual modal w/ bg and corners and stuff
 
 
 // Kill the scroll on the body
@@ -22,7 +22,9 @@
   // Prevent Chrome on Windows from adding a focus outline. For details, see
   // https://github.com/twbs/bootstrap/pull/10951.
   outline: 0;
-  -webkit-overflow-scrolling: touch;
+  // We deliberately don't use `-webkit-overflow-scrolling: touch;` due to a
+  // gnarly iOS Safari bug: https://bugs.webkit.org/show_bug.cgi?id=158342
+  // See also https://github.com/twbs/bootstrap/issues/17695
 
   // When fading in the modal, animate it to slide down
   &.fade .modal-dialog {
diff --git a/scss/_navbar.scss b/scss/_navbar.scss
index 9d3db1b26..cdc21e78c 100644
--- a/scss/_navbar.scss
+++ b/scss/_navbar.scss
@@ -105,10 +105,13 @@
 // Bootstrap JavaScript plugin.
 
 .navbar-toggler {
+  width: 2.5em;
+  height: 2em;
   padding: $navbar-toggler-padding-y $navbar-toggler-padding-x;
   font-size: $navbar-toggler-font-size;
   line-height: 1;
-  background: none;
+  background: transparent no-repeat center center;
+  background-size: 24px 24px;
   border: $border-width solid transparent;
   @include border-radius($navbar-toggler-border-radius);
 
@@ -117,28 +120,6 @@
   }
 }
 
-// scss-lint:disable ImportantRule
-// Custom override for
-.navbar-toggleable {
-  &-xs {
-    @include media-breakpoint-up(sm) {
-      display: block !important;
-    }
-  }
-  &-sm {
-    @include media-breakpoint-up(md) {
-      display: block !important;
-    }
-  }
-  &-md {
-    @include media-breakpoint-up(lg) {
-      display: block !important;
-    }
-  }
-}
-// scss-lint:enable ImportantRule
-
-
 // Navigation
 //
 // Custom navbar navigation built on the base `.nav` styles.
@@ -193,6 +174,10 @@
     }
   }
 
+  .navbar-toggler {
+    background-image: $navbar-light-toggler-bg;
+  }
+
   .navbar-divider {
     background-color: rgba(0,0,0,.075);
   }
@@ -228,7 +213,57 @@
     }
   }
 
+  .navbar-toggler {
+    background-image: $navbar-dark-toggler-bg;
+  }
+
   .navbar-divider {
     background-color: rgba(255,255,255,.075);
   }
 }
+
+
+// Navbar toggleable
+//
+// Custom override for collapse plugin in navbar.
+
+.navbar-toggleable {
+  &-xs {
+    @include clearfix;
+    @include media-breakpoint-down(xs) {
+      .navbar-nav .nav-item {
+        float: none;
+        margin-left: 0;
+      }
+    }
+    @include media-breakpoint-up(sm) {
+      display: block !important;
+    }
+  }
+
+  &-sm {
+    @include clearfix;
+    @include media-breakpoint-down(sm) {
+      .navbar-nav .nav-item {
+        float: none;
+        margin-left: 0;
+      }
+    }
+    @include media-breakpoint-up(md) {
+      display: block !important;
+    }
+  }
+
+  &-md {
+    @include clearfix;
+    @include media-breakpoint-down(md) {
+      .navbar-nav .nav-item {
+        float: none;
+        margin-left: 0;
+      }
+    }
+    @include media-breakpoint-up(lg) {
+      display: block !important;
+    }
+  }
+}
diff --git a/scss/_normalize.scss b/scss/_normalize.scss
index 8a74836b0..827b7f31a 100644
--- a/scss/_normalize.scss
+++ b/scss/_normalize.scss
@@ -1,14 +1,16 @@
-/*! normalize.css v4.0.0 | MIT License | github.com/necolas/normalize.css */
+/*! normalize.css v4.2.0 | MIT License | github.com/necolas/normalize.css */
 
 //
 // 1. Change the default font family in all browsers (opinionated).
-// 2. Prevent adjustments of font size after orientation changes in IE and iOS.
+// 2. Correct the line height in all browsers.
+// 3. Prevent adjustments of font size after orientation changes in IE and iOS.
 //
 
 html {
   font-family: sans-serif; // 1
-  -ms-text-size-adjust: 100%; // 2
-  -webkit-text-size-adjust: 100%; // 2
+  line-height: 1.15; // 2
+  -ms-text-size-adjust: 100%; // 3
+  -webkit-text-size-adjust: 100%; // 3
 }
 
 //
@@ -76,7 +78,7 @@ progress {
 // 1. Add the correct display in IE.
 //
 
-template, // 2
+template, // 1
 [hidden] {
   display: none;
 }
@@ -85,11 +87,13 @@ template, // 2
 // ==========================================================================
 
 //
-// Remove the gray background on active links in IE 10.
+// 1. Remove the gray background on active links in IE 10.
+// 2. Remove gaps in links underline in iOS 8+ and Safari 8+.
 //
 
 a {
-  background-color: transparent;
+  background-color: transparent; // 1
+  -webkit-text-decoration-skip: objects; // 2
 }
 
 //
@@ -248,14 +252,17 @@ hr {
 // ==========================================================================
 
 //
-// Change font properties to `inherit` in all browsers (opinionated).
+// 1. Change font properties to `inherit` in all browsers (opinionated).
+// 2. Remove the margin in Firefox and Safari.
 //
 
 button,
 input,
+optgroup,
 select,
 textarea {
-  font: inherit;
+  font: inherit; // 1
+  margin: 0; // 2
 }
 
 //
@@ -269,30 +276,16 @@ optgroup {
 //
 // Show the overflow in IE.
 // 1. Show the overflow in Edge.
-// 2. Show the overflow in Edge, Firefox, and IE.
 //
 
 button,
-input, // 1
-select { // 2
+input { // 1
   overflow: visible;
 }
 
 //
-// Remove the margin in Safari.
-// 1. Remove the margin in Firefox and Safari.
-//
-
-button,
-input,
-select,
-textarea { // 1
-  margin: 0;
-}
-
-//
-// Remove the inheritence of text transform in Edge, Firefox, and IE.
-// 1. Remove the inheritence of text transform in Firefox.
+// Remove the inheritance of text transform in Edge, Firefox, and IE.
+// 1. Remove the inheritance of text transform in Firefox.
 //
 
 button,
@@ -301,28 +294,9 @@ select { // 1
 }
 
 //
-// Change the cursor in all browsers (opinionated).
-//
-
-button,
-[type="button"],
-[type="reset"],
-[type="submit"] {
-  cursor: pointer;
-}
-
-//
-// Restore the default cursor to disabled elements unset by the previous rule.
-//
-
-[disabled] {
-  cursor: default;
-}
-
-//
 // 1. Prevent a WebKit bug where (2) destroys native `audio` and `video`
 //    controls in Android 4.
-// 2. Correct the inability to style clickable types in iOS.
+// 2. Correct the inability to style clickable types in iOS and Safari.
 //
 
 button,
@@ -337,8 +311,10 @@ html [type="button"], // 1
 //
 
 button::-moz-focus-inner,
-input::-moz-focus-inner {
-  border: 0;
+[type="button"]::-moz-focus-inner,
+[type="reset"]::-moz-focus-inner,
+[type="submit"]::-moz-focus-inner {
+  border-style: none;
   padding: 0;
 }
 
@@ -347,7 +323,9 @@ input::-moz-focus-inner {
 //
 
 button:-moz-focusring,
-input:-moz-focusring {
+[type="button"]:-moz-focusring,
+[type="reset"]:-moz-focusring,
+[type="submit"]:-moz-focusring {
   outline: 1px dotted ButtonText;
 }
 
@@ -406,19 +384,39 @@ textarea {
 }
 
 //
-// Correct the odd appearance of search inputs in Chrome and Safari.
+// 1. Correct the odd appearance in Chrome and Safari.
+// 2. Correct the outline style in Safari.
 //
 
 [type="search"] {
-  -webkit-appearance: textfield;
+  -webkit-appearance: textfield; // 1
+  outline-offset: -2px; // 2
 }
 
 //
-// Remove the inner padding and cancel buttons in Chrome on OS X and
-// Safari on OS X.
+// Remove the inner padding and cancel buttons in Chrome and Safari on OS X.
 //
 
 [type="search"]::-webkit-search-cancel-button,
 [type="search"]::-webkit-search-decoration {
   -webkit-appearance: none;
 }
+
+//
+// Correct the text style of placeholders in Chrome, Edge, and Safari.
+//
+
+::-webkit-input-placeholder {
+  color: inherit;
+  opacity: 0.54;
+}
+
+//
+// 1. Correct the inability to style clickable types in iOS and Safari.
+// 2. Change font properties to `inherit` in Safari.
+//
+
+::-webkit-file-upload-button {
+  -webkit-appearance: button; // 1
+  font: inherit; // 2
+}
diff --git a/scss/_popover.scss b/scss/_popover.scss
index 9ea8c1475..86b489a48 100644
--- a/scss/_popover.scss
+++ b/scss/_popover.scss
@@ -5,7 +5,7 @@
   z-index: $zindex-popover;
   display: block;
   max-width: $popover-max-width;
-  padding: 1px;
+  padding: $popover-inner-padding;
   // Our parent element can be arbitrary since tooltips are by default inserted as a sibling of their target element.
   // So reset our font and text properties to avoid inheriting weird values.
   @include reset-text();
@@ -25,19 +25,22 @@
   &.bs-tether-element-attached-bottom {
     margin-top: -$popover-arrow-width;
 
-    .popover-arrow {
-      bottom: -$popover-arrow-outer-width;
+    &::before,
+    &::after {
       left: 50%;
+      border-bottom-width: 0;
+    }
+
+    &::before {
+      bottom: -$popover-arrow-outer-width;
       margin-left: -$popover-arrow-outer-width;
       border-top-color: $popover-arrow-outer-color;
-      border-bottom-width: 0;
-      &::after {
-        bottom: 1px;
-        margin-left: -$popover-arrow-width;
-        content: "";
-        border-top-color: $popover-arrow-color;
-        border-bottom-width: 0;
-      }
+    }
+
+    &::after {
+      bottom: -($popover-arrow-outer-width - 1);
+      margin-left: -$popover-arrow-width;
+      border-top-color: $popover-arrow-color;
     }
   }
 
@@ -45,19 +48,22 @@
   &.bs-tether-element-attached-left {
     margin-left: $popover-arrow-width;
 
-    .popover-arrow {
+    &::before,
+    &::after {
       top: 50%;
+      border-left-width: 0;
+    }
+
+    &::before {
       left: -$popover-arrow-outer-width;
       margin-top: -$popover-arrow-outer-width;
       border-right-color: $popover-arrow-outer-color;
-      border-left-width: 0;
-      &::after {
-        bottom: -$popover-arrow-width;
-        left: 1px;
-        content: "";
-        border-right-color: $popover-arrow-color;
-        border-left-width: 0;
-      }
+    }
+
+    &::after {
+      left: -($popover-arrow-outer-width - 1);
+      margin-top: -($popover-arrow-outer-width - 1);
+      border-right-color: $popover-arrow-color;
     }
   }
 
@@ -65,19 +71,34 @@
   &.bs-tether-element-attached-top {
     margin-top: $popover-arrow-width;
 
-    .popover-arrow {
-      top: -$popover-arrow-outer-width;
+    &::before,
+    &::after {
       left: 50%;
-      margin-left: -$popover-arrow-outer-width;
       border-top-width: 0;
+    }
+
+    &::before {
+      top: -$popover-arrow-outer-width;
+      margin-left: -$popover-arrow-outer-width;
       border-bottom-color: $popover-arrow-outer-color;
-      &::after {
-        top: 1px;
-        margin-left: -$popover-arrow-width;
-        content: "";
-        border-top-width: 0;
-        border-bottom-color: $popover-arrow-color;
-      }
+    }
+
+    &::after {
+      top: -($popover-arrow-outer-width - 1);
+      margin-left: -$popover-arrow-width;
+      border-bottom-color: $popover-title-bg;
+    }
+
+    // This will remove the popover-title's border just below the arrow
+    .popover-title::before {
+      position: absolute;
+      top: 0;
+      left: 50%;
+      display: block;
+      width: 20px;
+      margin-left: -10px;
+      content: "";
+      border-bottom: 1px solid $popover-title-bg;
     }
   }
 
@@ -85,19 +106,22 @@
   &.bs-tether-element-attached-right {
     margin-left: -$popover-arrow-width;
 
-    .popover-arrow {
+    &::before,
+    &::after {
       top: 50%;
+      border-right-width: 0;
+    }
+
+    &::before {
       right: -$popover-arrow-outer-width;
       margin-top: -$popover-arrow-outer-width;
-      border-right-width: 0;
       border-left-color: $popover-arrow-outer-color;
-      &::after {
-        right: 1px;
-        bottom: -$popover-arrow-width;
-        content: "";
-        border-right-width: 0;
-        border-left-color: $popover-arrow-color;
-      }
+    }
+
+    &::after {
+      right: -($popover-arrow-outer-width - 1);
+      margin-top: -($popover-arrow-outer-width - 1);
+      border-left-color: $popover-arrow-color;
     }
   }
 }
@@ -112,6 +136,10 @@
   border-bottom: $popover-border-width solid darken($popover-title-bg, 5%);
   $offset-border-width: ($border-width / $font-size-root);
   @include border-radius(($border-radius-lg - $offset-border-width) ($border-radius-lg - $offset-border-width) 0 0);
+
+  &:empty {
+    display: none;
+  }
 }
 
 .popover-content {
@@ -123,21 +151,21 @@
 //
 // .popover-arrow is outer, .popover-arrow::after is inner
 
-.popover-arrow {
-  &,
-  &::after {
-    position: absolute;
-    display: block;
-    width: 0;
-    height: 0;
-    border-color: transparent;
-    border-style: solid;
-  }
+.popover::before,
+.popover::after {
+  position: absolute;
+  display: block;
+  width: 0;
+  height: 0;
+  border-color: transparent;
+  border-style: solid;
 }
-.popover-arrow {
+
+.popover::before {
+  content: "";
   border-width: $popover-arrow-outer-width;
 }
-.popover-arrow::after {
+.popover::after {
   content: "";
   border-width: $popover-arrow-width;
 }
diff --git a/scss/_print.scss b/scss/_print.scss
index b2c93735c..bd41328c2 100644
--- a/scss/_print.scss
+++ b/scss/_print.scss
@@ -1,4 +1,4 @@
-// scss-lint:disable ImportantRule, QualifyingElement
+// scss-lint:disable QualifyingElement
 
 // Source: https://github.com/h5bp/html5-boilerplate/blob/master/src/css/main.css
 
@@ -14,7 +14,10 @@
     *::before,
     *::after,
     *::first-letter,
-    *::first-line {
+    p::first-line,
+    div::first-line,
+    blockquote::first-line,
+    li::first-line {
       // Bootstrap specific; comment out `color` and `background`
       //color: #000 !important; // Black prints faster:
                                 //   http://www.sanbeiji.com/archives/953
@@ -45,9 +48,12 @@
 
     //a[href^="#"]::after,
     //a[href^="javascript:"]::after {
-    //  content: "";
+    // content: "";
     //}
 
+    pre {
+      white-space: pre-wrap !important;
+    }
     pre,
     blockquote {
       border: $border-width solid #999;   // Bootstrap custom code; using `$border-width` instead of 1px
diff --git a/scss/_progress.scss b/scss/_progress.scss
index b2bcb3056..c2364feee 100644
--- a/scss/_progress.scss
+++ b/scss/_progress.scss
@@ -26,7 +26,7 @@
   // Reset the default appearance
   appearance: none;
   // Set overall border radius
-  @include border-radius($border-radius);
+  @include border-radius($progress-border-radius);
 }
 
 // Filled-in portion of the bar
@@ -37,30 +37,30 @@
 }
 .progress[value]::-moz-progress-bar {
   background-color: $progress-bar-color;
-  @include border-left-radius($border-radius);
+  @include border-left-radius($progress-border-radius);
 }
 .progress[value]::-webkit-progress-value {
   background-color: $progress-bar-color;
-  @include border-left-radius($border-radius);
+  @include border-left-radius($progress-border-radius);
 }
 // Tweaks for full progress bar
 .progress[value="100"]::-moz-progress-bar {
-  @include border-right-radius($border-radius);
+  @include border-right-radius($progress-border-radius);
 }
 .progress[value="100"]::-webkit-progress-value {
-  @include border-right-radius($border-radius);
+  @include border-right-radius($progress-border-radius);
 }
 
 // Unfilled portion of the bar
 .progress[value]::-webkit-progress-bar {
   background-color: $progress-bg;
-  @include border-radius($border-radius);
+  @include border-radius($progress-border-radius);
   @include box-shadow($progress-box-shadow);
 }
 base::-moz-progress-bar, // Absurd-but-syntactically-valid selector to make these styles Firefox-only
 .progress[value] {
   background-color: $progress-bg;
-  @include border-radius($border-radius);
+  @include border-radius($progress-border-radius);
   @include box-shadow($progress-box-shadow);
 }
 
@@ -68,7 +68,7 @@ base::-moz-progress-bar, // Absurd-but-syntactically-valid selector to make thes
 @media screen and (min-width:0\0) {
   .progress {
     background-color: $progress-bg;
-    @include border-radius($border-radius);
+    @include border-radius($progress-border-radius);
     @include box-shadow($progress-box-shadow);
   }
   .progress-bar {
@@ -76,10 +76,10 @@ base::-moz-progress-bar, // Absurd-but-syntactically-valid selector to make thes
     height: $spacer-y;
     text-indent: -999rem; // Simulate hiding of value as in native `<progress>`
     background-color: $progress-bar-color;
-    @include border-left-radius($border-radius);
+    @include border-left-radius($progress-border-radius);
   }
   .progress[width="100%"] {
-    @include border-right-radius($border-radius);
+    @include border-right-radius($progress-border-radius);
   }
 }
 
diff --git a/scss/_reboot.scss b/scss/_reboot.scss
index e30de2c1b..461ca40e6 100644
--- a/scss/_reboot.scss
+++ b/scss/_reboot.scss
@@ -1,4 +1,4 @@
-// scss-lint:disable ImportantRule, QualifyingElement, DuplicateProperty
+// scss-lint:disable QualifyingElement, DuplicateProperty
 
 // Reboot
 //
@@ -15,7 +15,7 @@
 //
 // Heads up! This reset may cause conflicts with some third-party widgets. For
 // recommendations on resolving such conflicts, see
-// http://getbootstrap.com/getting-started/#third-box-sizing.
+// https://getbootstrap.com/getting-started/#third-box-sizing.
 //
 // Credit: https://css-tricks.com/inheriting-box-sizing-probably-slightly-better-best-practice/
 
@@ -33,15 +33,15 @@ html {
 // Make viewport responsive
 //
 // @viewport is needed because IE 10+ doesn't honor <meta name="viewport"> in
-// some cases. See http://timkadlec.com/2012/10/ie10-snap-mode-and-responsive-design/.
+// some cases. See https://timkadlec.com/2012/10/ie10-snap-mode-and-responsive-design/.
 // Eventually @viewport will replace <meta name="viewport">.
 //
 // However, `device-width` is broken on IE 10 on Windows (Phone) 8,
-// (see http://timkadlec.com/2013/01/windows-phone-8-and-device-width/ and https://github.com/twbs/bootstrap/issues/10497)
+// (see https://timkadlec.com/2013/01/windows-phone-8-and-device-width/ and https://github.com/twbs/bootstrap/issues/10497)
 // and the fix for that involves a snippet of JavaScript to sniff the user agent
 // and apply some conditional CSS.
 //
-// See http://getbootstrap.com/getting-started/#support-ie10-width for the relevant hack.
+// See https://getbootstrap.com/getting-started/#support-ie10-width for the relevant hack.
 //
 // Wrap `@viewport` with `@at-root` for when folks do a nested import (e.g.,
 // `.class-name { @import "bootstrap"; }`).
@@ -171,12 +171,13 @@ a {
   }
 }
 
-// And undo these styles for placeholder links/named anchors (without href).
+// And undo these styles for placeholder links/named anchors (without href)
+// which have not been made explicitly keyboard-focusable (without tabindex).
 // It would be more straightforward to just use a[href] in previous block, but that
 // causes specificity issues in many other styles that are too complex to fix.
 // See https://github.com/twbs/bootstrap/issues/19402
 
-a:not([href]) {
+a:not([href]):not([tabindex]) {
   color: inherit;
   text-decoration: none;
 
@@ -246,9 +247,9 @@ img {
 // DON'T remove the click delay when `<meta name="viewport" content="width=device-width">` is present.
 // However, they DO support removing the click delay via `touch-action: manipulation`.
 // See:
-// * http://v4-alpha.getbootstrap.com/content/reboot/#click-delay-optimization-for-touch
+// * https://v4-alpha.getbootstrap.com/content/reboot/#click-delay-optimization-for-touch
 // * http://caniuse.com/#feat=css-touch-action
-// * http://patrickhlauke.github.io/touch/tests/results/#suppressing-300ms-delay
+// * https://patrickhlauke.github.io/touch/tests/results/#suppressing-300ms-delay
 
 a,
 area,
@@ -311,14 +312,9 @@ input,
 button,
 select,
 textarea {
-  // Remove all `margin`s so our classes don't have to do it themselves.
-  margin: 0;
   // Normalize includes `font: inherit;`, so `font-family`. `font-size`, etc are
-  // properly inherited. However, `line-height` isn't addressed there. Using this
-  // ensures we don't need to unnecessarily redeclare the global font stack.
+  // properly inherited. However, `line-height` isn't inherited there.
   line-height: inherit;
-  // iOS adds rounded borders by default
-  border-radius: 0;
 }
 
 input[type="radio"],
diff --git a/scss/_tables.scss b/scss/_tables.scss
index d865884ec..aeedf0b69 100644
--- a/scss/_tables.scss
+++ b/scss/_tables.scss
@@ -101,7 +101,7 @@
 
 // Inverse styles
 //
-// Same table markup, but inverted color scheme—dark background and light text.
+// Same table markup, but inverted color scheme: dark background and light text.
 
 .thead-inverse {
   th {
@@ -143,7 +143,7 @@
 .table-responsive {
   display: block;
   width: 100%;
-  min-height: .01%; // Workaround for IE9 bug (see https://github.com/twbs/bootstrap/issues/14837)
+  min-height: 0%; // Workaround for IE9 bug (see https://github.com/twbs/bootstrap/issues/14837)
   overflow-x: auto;
 
   // TODO: find out if we need this still.
@@ -184,7 +184,6 @@
     }
   }
 
-  // scss-lint:disable ImportantRule
   tr {
     float: left;
 
@@ -194,5 +193,4 @@
       border: $table-border-width solid $table-border-color;
     }
   }
-  // scss-lint:enable ImportantRule
 }
diff --git a/scss/_tooltip.scss b/scss/_tooltip.scss
index e8151af07..57ba8cf0c 100644
--- a/scss/_tooltip.scss
+++ b/scss/_tooltip.scss
@@ -18,10 +18,11 @@
     padding: $tooltip-arrow-width 0;
     margin-top: -$tooltip-margin;
 
-    .tooltip-arrow {
+    .tooltip-inner::before {
       bottom: 0;
       left: 50%;
       margin-left: -$tooltip-arrow-width;
+      content: "";
       border-width: $tooltip-arrow-width $tooltip-arrow-width 0;
       border-top-color: $tooltip-arrow-color;
     }
@@ -31,10 +32,11 @@
     padding: 0 $tooltip-arrow-width;
     margin-left: $tooltip-margin;
 
-    .tooltip-arrow {
+    .tooltip-inner::before {
       top: 50%;
       left: 0;
       margin-top: -$tooltip-arrow-width;
+      content: "";
       border-width: $tooltip-arrow-width $tooltip-arrow-width $tooltip-arrow-width 0;
       border-right-color: $tooltip-arrow-color;
     }
@@ -44,10 +46,11 @@
     padding: $tooltip-arrow-width 0;
     margin-top: $tooltip-margin;
 
-    .tooltip-arrow {
+    .tooltip-inner::before {
       top: 0;
       left: 50%;
       margin-left: -$tooltip-arrow-width;
+      content: "";
       border-width: 0 $tooltip-arrow-width $tooltip-arrow-width;
       border-bottom-color: $tooltip-arrow-color;
     }
@@ -57,10 +60,11 @@
     padding: 0 $tooltip-arrow-width;
     margin-left: -$tooltip-margin;
 
-    .tooltip-arrow {
+    .tooltip-inner::before {
       top: 50%;
       right: 0;
       margin-top: -$tooltip-arrow-width;
+      content: "";
       border-width: $tooltip-arrow-width 0 $tooltip-arrow-width $tooltip-arrow-width;
       border-left-color: $tooltip-arrow-color;
     }
@@ -75,13 +79,12 @@
   text-align: center;
   background-color: $tooltip-bg;
   @include border-radius($border-radius);
-}
 
-// Arrows
-.tooltip-arrow {
-  position: absolute;
-  width: 0;
-  height: 0;
-  border-color: transparent;
-  border-style: solid;
+  &::before {
+    position: absolute;
+    width: 0;
+    height: 0;
+    border-color: transparent;
+    border-style: solid;
+  }
 }
diff --git a/scss/_utilities.scss b/scss/_utilities.scss
index 87b8ba6da..aef99aeba 100644
--- a/scss/_utilities.scss
+++ b/scss/_utilities.scss
@@ -1,8 +1,9 @@
 @import "utilities/background";
+@import "utilities/borders";
 @import "utilities/clearfix";
 @import "utilities/display";
 @import "utilities/flex";
-@import "utilities/pulls";
+@import "utilities/float";
 @import "utilities/screenreaders";
 @import "utilities/spacing";
 @import "utilities/text";
diff --git a/scss/_variables.scss b/scss/_variables.scss
index 2dcfe8b88..e8a18790b 100644
--- a/scss/_variables.scss
+++ b/scss/_variables.scss
@@ -21,7 +21,11 @@
   $prev-key: null;
   $prev-num: null;
   @each $key, $num in $map {
-    @if $prev-num != null and $prev-num >= $num {
+    @if $prev-num == null {
+      // Do nothing
+    } @else if not comparable($prev-num, $num) {
+      @warn "Potentially invalid value for #{$map-name}: This map must be in ascending order, but key '#{$key}' has value #{$num} whose unit makes it incomparable to #{$prev-num}, the value of the previous key '#{$prev-key}' !";
+    } @else if $prev-num >= $num {
       @warn "Invalid value for #{$map-name}: This map must be in ascending order, but key '#{$key}' has value #{$num} which isn't greater than #{$prev-num}, the value of the previous key '#{$prev-key}' !";
     }
     $prev-key: $key;
@@ -121,7 +125,7 @@ $link-hover-decoration: underline !default;
 
 $grid-breakpoints: (
   xs: 0,
-  sm: 544px,
+  sm: 576px,
   md: 768px,
   lg: 992px,
   xl: 1200px
@@ -134,9 +138,9 @@ $grid-breakpoints: (
 // Define the maximum width of `.container` for different screen sizes.
 
 $container-max-widths: (
-  sm: 576px,
+  sm: 540px,
   md: 720px,
-  lg: 940px,
+  lg: 960px,
   xl: 1140px
 ) !default;
 @include _assert-ascending($container-max-widths, "$container-max-widths");
@@ -146,15 +150,21 @@ $container-max-widths: (
 //
 // Set the number of columns and specify the width of the gutters.
 
-$grid-columns:      12 !default;
-$grid-gutter-width: 30px !default;
-
+$grid-columns:               12 !default;
+$grid-gutter-width-base:     30px !default;
+$grid-gutter-widths: (
+  xs: $grid-gutter-width-base,
+  sm: $grid-gutter-width-base,
+  md: $grid-gutter-width-base,
+  lg: $grid-gutter-width-base,
+  xl: $grid-gutter-width-base
+) !default;
 
 // Typography
 //
 // Font, line-height, and color for body text, headings, and more.
 
-$font-family-sans-serif: -apple-system, BlinkMacSystemFont, "Segoe UI", "Roboto", "Oxygen", "Ubuntu", "Cantarell", "Fira Sans", "Droid Sans", "Helvetica Neue", Arial, sans-serif !default;
+$font-family-sans-serif: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif !default;
 $font-family-serif:      Georgia, "Times New Roman", Times, serif !default;
 $font-family-monospace:  Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace !default;
 $font-family-base:       $font-family-sans-serif !default;
@@ -323,10 +333,10 @@ $input-border-radius:            $border-radius !default;
 $input-border-radius-lg:         $border-radius-lg !default;
 $input-border-radius-sm:         $border-radius-sm !default;
 
-$input-bg-focus:                 $input-bg;
+$input-bg-focus:                 $input-bg !default;
 $input-border-focus:             #66afe9 !default;
-$input-box-shadow-focus:         rgba(102,175,233,.6) !default;
-$input-color-focus:              $input-color;
+$input-box-shadow-focus:         $input-box-shadow, 0 0 8px rgba(102,175,233,.6) !default;
+$input-color-focus:              $input-color !default;
 
 $input-color-placeholder:        #999 !default;
 
@@ -371,13 +381,15 @@ $custom-control-active-indicator-bg:         #84c6ff !default;
 $custom-control-active-indicator-box-shadow: none !default;
 
 $custom-checkbox-radius: $border-radius !default;
-$custom-checkbox-checked-icon: url("data:image/svg+xml;charset=utf8,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 8 8'%3E%3Cpath fill='%23fff' d='M6.564.75l-3.59 3.612-1.538-1.55L0 4.26 2.974 7.25 8 2.193z'/%3E%3C/svg%3E") !default;
+$custom-checkbox-checked-icon: url("data:image/svg+xml;charset=utf8,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 8 8'%3E%3Cpath fill='#{$custom-control-checked-indicator-color}' d='M6.564.75l-3.59 3.612-1.538-1.55L0 4.26 2.974 7.25 8 2.193z'/%3E%3C/svg%3E") !default;
+
 $custom-checkbox-indeterminate-bg: #0074d9 !default;
-$custom-checkbox-indeterminate-icon: url("data:image/svg+xml;charset=utf8,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 4 4'%3E%3Cpath stroke='%23fff' d='M0 2h4'/%3E%3C/svg%3E") !default;
+$custom-checkbox-indeterminate-indicator-color: $custom-control-checked-indicator-color !default;
+$custom-checkbox-indeterminate-icon: url("data:image/svg+xml;charset=utf8,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 4 4'%3E%3Cpath stroke='#{$custom-checkbox-indeterminate-indicator-color}' d='M0 2h4'/%3E%3C/svg%3E") !default;
 $custom-checkbox-indeterminate-box-shadow: none !default;
 
 $custom-radio-radius: 50% !default;
-$custom-radio-checked-icon: url("data:image/svg+xml;charset=utf8,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='-4 -4 8 8'%3E%3Ccircle r='3' fill='%23fff'/%3E%3C/svg%3E") !default;
+$custom-radio-checked-icon: url("data:image/svg+xml;charset=utf8,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='-4 -4 8 8'%3E%3Ccircle r='3' fill='#{$custom-control-checked-indicator-color}'/%3E%3C/svg%3E") !default;
 
 $custom-select-padding-x:          .75rem  !default;
 $custom-select-padding-y:          .375rem !default;
@@ -387,7 +399,8 @@ $custom-select-disabled-color: $gray-light !default;
 $custom-select-bg:            #fff !default;
 $custom-select-disabled-bg:   $gray-lighter !default;
 $custom-select-bg-size:       8px 10px !default; // In pixels because image dimensions
-$custom-select-indicator:     url("data:image/svg+xml;charset=utf8,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 4 5'%3E%3Cpath fill='%23333' d='M2 0L0 2h4zm0 5L0 3h4z'/%3E%3C/svg%3E") !default;
+$custom-select-indicator-color: #333 !default;
+$custom-select-indicator:     url("data:image/svg+xml;charset=utf8,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 4 5'%3E%3Cpath fill='#{$custom-select-indicator-color}' d='M2 0L0 2h4zm0 5L0 3h4z'/%3E%3C/svg%3E") !default;
 $custom-select-border-width:  $input-btn-border-width !default;
 $custom-select-border-color:  $input-border-color !default;
 $custom-select-border-radius: $border-radius !default;
@@ -424,23 +437,28 @@ $custom-file-text: (
 
 
 // Form validation icons
-$form-icon-success: url("data:image/svg+xml;charset=utf8,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 8 8'%3E%3Cpath fill='%235cb85c' d='M2.3 6.73L.6 4.53c-.4-1.04.46-1.4 1.1-.8l1.1 1.4 3.4-3.8c.6-.63 1.6-.27 1.2.7l-4 4.6c-.43.5-.8.4-1.1.1z'/%3E%3C/svg%3E") !default;
-$form-icon-warning: url("data:image/svg+xml;charset=utf8,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 8 8'%3E%3Cpath fill='%23f0ad4e' d='M4.4 5.324h-.8v-2.46h.8zm0 1.42h-.8V5.89h.8zM3.76.63L.04 7.075c-.115.2.016.425.26.426h7.397c.242 0 .372-.226.258-.426C6.726 4.924 5.47 2.79 4.253.63c-.113-.174-.39-.174-.494 0z'/%3E%3C/svg%3E") !default;
-$form-icon-danger: url("data:image/svg+xml;charset=utf8,%3Csvg xmlns='http://www.w3.org/2000/svg' fill='%23d9534f' viewBox='-2 -2 7 7'%3E%3Cpath stroke='%23d9534f' d='M0 0l3 3m0-3L0 3'/%3E%3Ccircle r='.5'/%3E%3Ccircle cx='3' r='.5'/%3E%3Ccircle cy='3' r='.5'/%3E%3Ccircle cx='3' cy='3' r='.5'/%3E%3C/svg%3E") !default;
+$form-icon-success-color: $brand-success !default;
+$form-icon-success: url("data:image/svg+xml;charset=utf8,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 8 8'%3E%3Cpath fill='#{$form-icon-success-color}' d='M2.3 6.73L.6 4.53c-.4-1.04.46-1.4 1.1-.8l1.1 1.4 3.4-3.8c.6-.63 1.6-.27 1.2.7l-4 4.6c-.43.5-.8.4-1.1.1z'/%3E%3C/svg%3E") !default;
+
+$form-icon-warning-color: $brand-warning !default;
+$form-icon-warning: url("data:image/svg+xml;charset=utf8,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 8 8'%3E%3Cpath fill='#{$form-icon-warning-color}' d='M4.4 5.324h-.8v-2.46h.8zm0 1.42h-.8V5.89h.8zM3.76.63L.04 7.075c-.115.2.016.425.26.426h7.397c.242 0 .372-.226.258-.426C6.726 4.924 5.47 2.79 4.253.63c-.113-.174-.39-.174-.494 0z'/%3E%3C/svg%3E") !default;
+
+$form-icon-danger-color: $brand-danger !default;
+$form-icon-danger: url("data:image/svg+xml;charset=utf8,%3Csvg xmlns='http://www.w3.org/2000/svg' fill='#{$form-icon-danger-color}' viewBox='-2 -2 7 7'%3E%3Cpath stroke='%23d9534f' d='M0 0l3 3m0-3L0 3'/%3E%3Ccircle r='.5'/%3E%3Ccircle cx='3' r='.5'/%3E%3Ccircle cy='3' r='.5'/%3E%3Ccircle cx='3' cy='3' r='.5'/%3E%3C/svg%3E") !default;
 
 
 // Dropdowns
 //
 // Dropdown menu container and contents.
 
-$dropdown-min-width:             160px !default;
-$dropdown-padding-y:             5px !default;
-$dropdown-margin-top:            2px !default;
+$dropdown-min-width:             10rem !default;
+$dropdown-padding-y:             .5rem !default;
+$dropdown-margin-top:            .125rem !default;
 $dropdown-bg:                    #fff !default;
 $dropdown-border-color:          rgba(0,0,0,.15) !default;
 $dropdown-border-width:          $border-width !default;
 $dropdown-divider-bg:            #e5e5e5 !default;
-$dropdown-box-shadow:            0 6px 12px rgba(0,0,0,.175) !default;
+$dropdown-box-shadow:            0 .5rem 1rem rgba(0,0,0,.175) !default;
 
 $dropdown-link-color:            $gray-dark !default;
 $dropdown-link-hover-color:      darken($gray-dark, 5%) !default;
@@ -451,7 +469,7 @@ $dropdown-link-active-bg:        $component-active-bg !default;
 
 $dropdown-link-disabled-color:   $gray-light !default;
 
-$dropdown-item-padding-x:        20px !default;
+$dropdown-item-padding-x:        1.5rem !default;
 
 $dropdown-header-color:          $gray-light !default;
 
@@ -484,16 +502,20 @@ $navbar-divider-padding-y:          .425rem !default;
 
 $navbar-toggler-font-size:           $font-size-lg !default;
 $navbar-toggler-border-radius:       $btn-border-radius !default;
+$navbar-toggler-padding-x:           .75rem !default;
+$navbar-toggler-padding-y:           .5rem !default;
 
 $navbar-dark-color:                 rgba(255,255,255,.5) !default;
 $navbar-dark-hover-color:           rgba(255,255,255,.75) !default;
 $navbar-dark-active-color:          rgba(255,255,255,1) !default;
 $navbar-dark-disabled-color:        rgba(255,255,255,.25) !default;
+$navbar-dark-toggler-bg: url("data:image/svg+xml;charset=utf8,%3Csvg viewBox='0 0 32 32' xmlns='http://www.w3.org/2000/svg'%3E%3Cpath stroke='#{$navbar-dark-color}' stroke-width='2' stroke-linecap='round' stroke-miterlimit='10' d='M4 8h24M4 16h24M4 24h24'/%3E%3C/svg%3E") !default;
 
 $navbar-light-color:                rgba(0,0,0,.3) !default;
 $navbar-light-hover-color:          rgba(0,0,0,.6) !default;
 $navbar-light-active-color:         rgba(0,0,0,.8) !default;
 $navbar-light-disabled-color:       rgba(0,0,0,.15) !default;
+$navbar-light-toggler-bg: url("data:image/svg+xml;charset=utf8,%3Csvg viewBox='0 0 32 32' xmlns='http://www.w3.org/2000/svg'%3E%3Cpath stroke='#{$navbar-light-color}' stroke-width='2' stroke-linecap='round' stroke-miterlimit='10' d='M4 8h24M4 16h24M4 24h24'/%3E%3C/svg%3E") !default;
 
 
 // Navs
@@ -583,7 +605,7 @@ $card-spacer-y:            .75rem !default;
 $card-border-width:        1px !default;
 $card-border-radius:       $border-radius !default;
 $card-border-color:        rgba(0,0,0,.125) !default;
-$card-border-radius-inner: $card-border-radius !default;
+$card-border-radius-inner: calc(#{$card-border-radius} - #{$card-border-width}) !default;
 $card-cap-bg:              #f5f5f5 !default;
 $card-bg:                  #fff !default;
 
@@ -612,6 +634,7 @@ $tooltip-arrow-color:         $tooltip-bg !default;
 
 // Popovers
 
+$popover-inner-padding:               1px !default;
 $popover-bg:                          #fff !default;
 $popover-max-width:                   276px !default;
 $popover-border-width:                $border-width !default;
@@ -686,7 +709,7 @@ $modal-sm:                    300px !default;
 //
 // Define alert colors, border radius, and padding.
 
-$alert-padding:               15px !default;
+$alert-padding:               1rem !default;
 $alert-border-radius:         $border-radius !default;
 $alert-link-font-weight:      bold !default;
 $alert-border-width:          $border-width !default;
diff --git a/scss/bootstrap-grid.scss b/scss/bootstrap-grid.scss
index 5dc331eb6..40680ca96 100644
--- a/scss/bootstrap-grid.scss
+++ b/scss/bootstrap-grid.scss
@@ -8,53 +8,13 @@
 // Variables
 //
 
-
-// Grid breakpoints
-//
-// Define the minimum and maximum dimensions at which your layout will change,
-// adapting to different screen sizes, for use in media queries.
-
-$grid-breakpoints: (
-  // Extra small screen / phone
-  xs: 0,
-  // Small screen / phone
-  sm: 544px,
-  // Medium screen / tablet
-  md: 768px,
-  // Large screen / desktop
-  lg: 992px,
-  // Extra large screen / wide desktop
-  xl: 1200px
-) !default;
-
-
-// Grid containers
-//
-// Define the maximum width of `.container` for different screen sizes.
-
-$container-max-widths: (
-  sm: 576px,
-  md: 720px,
-  lg: 940px,
-  xl: 1140px
-) !default;
-
-
-// Grid columns
-//
-// Set the number of columns and specify the width of the gutters.
-
-$grid-columns:               12 !default;
-$grid-gutter-width:          1.875rem !default; // 30px
-
+@import "custom";
+@import "variables";
 
 //
 // Grid mixins
 //
 
-@import "custom";
-@import "variables";
-
 @import "mixins/clearfix";
 @import "mixins/breakpoints";
 @import "mixins/grid-framework";
diff --git a/scss/bootstrap.scss b/scss/bootstrap.scss
index f99a722ca..ce61be8eb 100644
--- a/scss/bootstrap.scss
+++ b/scss/bootstrap.scss
@@ -1,5 +1,6 @@
 /*!
- * Bootstrap v4.0.0-alpha.2 (http://getbootstrap.com)
+ * Bootstrap v4.0.0-alpha.4 (https://getbootstrap.com)
+ * Copyright 2011-2016 The Bootstrap Authors
  * Copyright 2011-2016 Twitter, Inc.
  * Licensed under MIT (https://github.com/twbs/bootstrap/blob/master/LICENSE)
  */
diff --git a/scss/mixins/_background-variant.scss b/scss/mixins/_background-variant.scss
index ff21ad958..54a734dcc 100644
--- a/scss/mixins/_background-variant.scss
+++ b/scss/mixins/_background-variant.scss
@@ -2,7 +2,6 @@
 
 @mixin bg-variant($parent, $color) {
   #{$parent} {
-    color: #fff !important;
     background-color: $color !important;
   }
   a#{$parent} {
diff --git a/scss/mixins/_pulls.scss b/scss/mixins/_float.scss
index 6bdff025d..b30cf1468 100644
--- a/scss/mixins/_pulls.scss
+++ b/scss/mixins/_float.scss
@@ -1,6 +1,6 @@
-@mixin pull-left {
+@mixin float-left {
   float: left !important;
 }
-@mixin pull-right {
+@mixin float-right {
   float: right !important;
 }
diff --git a/scss/mixins/_forms.scss b/scss/mixins/_forms.scss
index 026e41e23..39851b8f8 100644
--- a/scss/mixins/_forms.scss
+++ b/scss/mixins/_forms.scss
@@ -7,26 +7,20 @@
   // Color the label and help text
   .form-control-feedback,
   .form-control-label,
-  .radio,
-  .checkbox,
-  .radio-inline,
-  .checkbox-inline,
-  &.radio label,
-  &.checkbox label,
-  &.radio-inline label,
-  &.checkbox-inline label,
+  .form-check-label,
+  .form-check-inline,
   .custom-control {
     color: $color;
   }
+
   // Set the border and box shadow on specific inputs to match
   .form-control {
     border-color: $color;
-    // @include box-shadow(inset 0 1px 1px rgba(0,0,0,.075)); // Redeclare so transitions work
 
-    &:focus {
-      // border-color: darken($border-color, 10%);
-      // $shadow: inset 0 1px 1px rgba(0,0,0,.075), 0 0 6px lighten($border-color, 20%);
-      // @include box-shadow($shadow);
+    @if $enable-rounded {
+      &:focus {
+        box-shadow: $input-box-shadow, 0 0 6px lighten($color, 20%);
+      }
     }
   }
 
@@ -36,10 +30,6 @@
     border-color: $color;
     background-color: lighten($color, 40%);
   }
-  // Optional feedback icon
-  .form-control-feedback {
-    color: $color;
-  }
 }
 
 // Form control focus state
@@ -60,8 +50,7 @@
     background-color: $input-bg-focus;
     border-color: $input-border-focus;
     outline: none;
-    $shadow: inset 0 1px 1px rgba(0,0,0,.075), 0 0 8px $input-box-shadow-focus;
-    @include box-shadow($shadow);
+    @include box-shadow($input-box-shadow-focus);
   }
 }
 
diff --git a/scss/mixins/_grid-framework.scss b/scss/mixins/_grid-framework.scss
index cb25be92a..35ccd9221 100644
--- a/scss/mixins/_grid-framework.scss
+++ b/scss/mixins/_grid-framework.scss
@@ -3,26 +3,48 @@
 // Used only by Bootstrap to generate the correct number of grid classes given
 // any value of `$grid-columns`.
 
-@mixin make-grid-columns($columns: $grid-columns, $gutter: $grid-gutter-width, $breakpoints: $grid-breakpoints) {
+@mixin make-grid-columns($columns: $grid-columns, $gutters: $grid-gutter-widths, $breakpoints: $grid-breakpoints) {
+  // Common properties for all breakpoints
+  %grid-column {
+    position: relative;
+    // Prevent columns from collapsing when empty
+    min-height: 1px;
+
+    @if $enable-flex {
+      width: 100%;
+    }
+
+    @include make-gutters($gutters);
+  }
+
   $breakpoint-counter: 0;
   @each $breakpoint in map-keys($breakpoints) {
     $breakpoint-counter: ($breakpoint-counter + 1);
+
+    // Allow columns to stretch full width below their breakpoints
+    .col-#{$breakpoint} {
+      @extend %grid-column;
+    }
+
+    @for $i from 1 through $columns {
+      .col-#{$breakpoint}-#{$i} {
+        @extend %grid-column;
+      }
+    }
+
     @include media-breakpoint-up($breakpoint, $breakpoints) {
+      // Provide basic `.col-{bp}` classes for equal-width flexbox columns
       @if $enable-flex {
         .col-#{$breakpoint} {
-          position: relative;
           flex-basis: 0;
           flex-grow: 1;
           max-width: 100%;
-          min-height: 1px;
-          padding-right: ($gutter / 2);
-          padding-left:  ($gutter / 2);
         }
       }
 
       @for $i from 1 through $columns {
         .col-#{$breakpoint}-#{$i} {
-          @include make-col($i, $columns, $gutter);
+          @include make-col($i, $columns);
         }
       }
 
diff --git a/scss/mixins/_grid.scss b/scss/mixins/_grid.scss
index 3ba4f43bc..88cbd249a 100644
--- a/scss/mixins/_grid.scss
+++ b/scss/mixins/_grid.scss
@@ -2,7 +2,7 @@
 //
 // Generate semantic grid columns with these mixins.
 
-@mixin make-container($gutter: $grid-gutter-width) {
+@mixin make-container($gutter: $grid-gutter-width-base) {
   margin-left: auto;
   margin-right: auto;
   padding-left:  ($gutter / 2);
@@ -17,28 +17,60 @@
 @mixin make-container-max-widths($max-widths: $container-max-widths, $breakpoints: $grid-breakpoints) {
   @each $breakpoint, $container-max-width in $max-widths {
     @include media-breakpoint-up($breakpoint, $breakpoints) {
-      max-width: $container-max-width;
+      width: $container-max-width;
+      max-width: 100%;
     }
   }
 }
 
-@mixin make-row($gutter: $grid-gutter-width) {
+@mixin make-gutters($gutters: $grid-gutter-widths) {
+  @each $breakpoint in map-keys($gutters) {
+    @include media-breakpoint-up($breakpoint) {
+      $gutter: map-get($gutters, $breakpoint);
+      padding-right: ($gutter / 2);
+      padding-left:  ($gutter / 2);
+    }
+  }
+}
+
+@mixin make-row($gutters: $grid-gutter-widths) {
   @if $enable-flex {
     display: flex;
     flex-wrap: wrap;
   } @else {
     @include clearfix();
   }
-  margin-left:  ($gutter / -2);
-  margin-right: ($gutter / -2);
+
+  @each $breakpoint in map-keys($gutters) {
+    @include media-breakpoint-up($breakpoint) {
+      $gutter: map-get($gutters, $breakpoint);
+      margin-right: ($gutter / -2);
+      margin-left:  ($gutter / -2);
+    }
+  }
 }
 
-@mixin make-col($size, $columns: $grid-columns, $gutter: $grid-gutter-width) {
+@mixin make-col-ready($gutters: $grid-gutter-widths) {
   position: relative;
-  min-height: 1px;
-  padding-right: ($gutter / 2);
-  padding-left:  ($gutter / 2);
+  min-height: 1px; // Prevent collapsing
+
+  // Prevent columns from becoming too narrow when at smaller grid tiers by
+  // always setting `width: 100%;`. This works because we use `flex` values
+  // later on to override this initial width.
+  @if $enable-flex {
+    width: 100%;
+  }
+
+  @each $breakpoint in map-keys($gutters) {
+    @include media-breakpoint-up($breakpoint) {
+      $gutter: map-get($gutters, $breakpoint);
+      padding-right: ($gutter / 2);
+      padding-left:  ($gutter / 2);
+    }
+  }
+}
 
+@mixin make-col($size, $columns: $grid-columns) {
   @if $enable-flex {
     flex: 0 0 percentage($size / $columns);
     // Add a `max-width` to ensure content within each column does not blow out
diff --git a/scss/mixins/_hover.scss b/scss/mixins/_hover.scss
index 3a11254e8..6dd55e705 100644
--- a/scss/mixins/_hover.scss
+++ b/scss/mixins/_hover.scss
@@ -1,14 +1,15 @@
 @mixin hover {
-  @if $enable-hover-media-query {
-    // See Media Queries Level 4: http://drafts.csswg.org/mediaqueries/#hover
-    // Currently shimmed by https://github.com/twbs/mq4-hover-shim
-    @media (hover: hover) {
-      &:hover { @content }
-    }
-  }
-  @else {
+  // TODO: re-enable along with mq4-hover-shim
+//  @if $enable-hover-media-query {
+//    // See Media Queries Level 4: https://drafts.csswg.org/mediaqueries/#hover
+//    // Currently shimmed by https://github.com/twbs/mq4-hover-shim
+//    @media (hover: hover) {
+//      &:hover { @content }
+//    }
+//  }
+//  @else {
     &:hover { @content }
-  }
+//  }
 }
 
 @mixin hover-focus {
diff --git a/scss/mixins/_image.scss b/scss/mixins/_image.scss
index 91d2f59ee..c2b45f2ce 100644
--- a/scss/mixins/_image.scss
+++ b/scss/mixins/_image.scss
@@ -7,10 +7,12 @@
 //
 // Keep images from scaling beyond the width of their parents.
 
-@mixin img-fluid($display: block) {
-  display: $display;
-  max-width: 100%; // Part 1: Set a maximum relative to the parent
-  height: auto; // Part 2: Scale the height according to the width, otherwise you get stretching
+@mixin img-fluid {
+  // Part 1: Set a maximum relative to the parent
+  max-width: 100%;
+  // Part 2: Override the height to auto, otherwise images will be stretched
+  // when setting a width and height attribute on the img element.
+  height: auto;
 }
 
 
diff --git a/scss/mixins/_screen-reader.scss b/scss/mixins/_screen-reader.scss
index 6ae65516a..c20858324 100644
--- a/scss/mixins/_screen-reader.scss
+++ b/scss/mixins/_screen-reader.scss
@@ -15,7 +15,7 @@
 
 // Use in conjunction with .sr-only to only display content when it's focused.
 //
-// Useful for "Skip to main content" links; see http://www.w3.org/TR/2013/NOTE-WCAG20-TECHS-20130905/G1
+// Useful for "Skip to main content" links; see https://www.w3.org/TR/2013/NOTE-WCAG20-TECHS-20130905/G1
 //
 // Credit: HTML5 Boilerplate
 
diff --git a/scss/mixins/_tab-focus.scss b/scss/mixins/_tab-focus.scss
index 7df0ae7ca..f16ed6428 100644
--- a/scss/mixins/_tab-focus.scss
+++ b/scss/mixins/_tab-focus.scss
@@ -1,9 +1,9 @@
 // WebKit-style focus
 
 @mixin tab-focus() {
-  // Default
-  outline: thin dotted;
-  // WebKit
+  // WebKit-specific. Other browsers will keep their default outline style.
+  // (Initially tried to also force default via `outline: initial`,
+  // but that seems to erroneously remove the outline in Firefox altogether.)
   outline: 5px auto -webkit-focus-ring-color;
   outline-offset: -2px;
 }
diff --git a/scss/mixins/_text-emphasis.scss b/scss/mixins/_text-emphasis.scss
index 27a4f454e..9cd4b6a4f 100644
--- a/scss/mixins/_text-emphasis.scss
+++ b/scss/mixins/_text-emphasis.scss
@@ -6,7 +6,7 @@
   }
   a#{$parent} {
     @include hover-focus {
-      color: darken($color, 10%);
+      color: darken($color, 10%) !important;
     }
   }
 }
diff --git a/scss/utilities/_background.scss b/scss/utilities/_background.scss
index b1589792f..a81dcff2c 100644
--- a/scss/utilities/_background.scss
+++ b/scss/utilities/_background.scss
@@ -2,10 +2,6 @@
 // Contextual backgrounds
 //
 
-.bg-inverse {
-  background-color: $brand-inverse;
-}
-
 .bg-faded {
   background-color: $gray-lightest;
 }
@@ -19,3 +15,5 @@
 @include bg-variant('.bg-warning', $brand-warning);
 
 @include bg-variant('.bg-danger', $brand-danger);
+
+@include bg-variant('.bg-inverse', $brand-inverse);
diff --git a/scss/utilities/_borders.scss b/scss/utilities/_borders.scss
new file mode 100644
index 000000000..b2e70aaf5
--- /dev/null
+++ b/scss/utilities/_borders.scss
@@ -0,0 +1,30 @@
+//
+// Border-width
+//
+
+// TBD...?
+
+
+//
+// Border-radius
+//
+
+.rounded {
+  @include border-radius($border-radius);
+}
+.rounded-top {
+  @include border-top-radius($border-radius);
+}
+.rounded-right {
+  @include border-right-radius($border-radius);
+}
+.rounded-bottom {
+  @include border-bottom-radius($border-radius);
+}
+.rounded-left {
+  @include border-left-radius($border-radius);
+}
+
+.rounded-circle {
+  border-radius: 50%;
+}
diff --git a/scss/utilities/_flex.scss b/scss/utilities/_flex.scss
index a61135ce2..502885a81 100644
--- a/scss/utilities/_flex.scss
+++ b/scss/utilities/_flex.scss
@@ -8,6 +8,7 @@
     @include media-breakpoint-up($breakpoint) {
       .flex-#{$breakpoint}-first { order: -1; }
       .flex-#{$breakpoint}-last { order: 1; }
+      .flex-#{$breakpoint}-unordered { order: 0; }
     }
 
     // Alignment for every item
diff --git a/scss/utilities/_float.scss b/scss/utilities/_float.scss
new file mode 100644
index 000000000..eea34bff2
--- /dev/null
+++ b/scss/utilities/_float.scss
@@ -0,0 +1,13 @@
+@each $breakpoint in map-keys($grid-breakpoints) {
+  @include media-breakpoint-up($breakpoint) {
+    .float-#{$breakpoint}-left {
+      @include float-left();
+    }
+    .float-#{$breakpoint}-right {
+      @include float-right();
+    }
+    .float-#{$breakpoint}-none {
+      float: none !important;
+    }
+  }
+}
diff --git a/scss/utilities/_pulls.scss b/scss/utilities/_pulls.scss
deleted file mode 100644
index 7fec56c9a..000000000
--- a/scss/utilities/_pulls.scss
+++ /dev/null
@@ -1,13 +0,0 @@
-@each $breakpoint in map-keys($grid-breakpoints) {
-  @include media-breakpoint-up($breakpoint) {
-    .pull-#{$breakpoint}-left {
-      @include pull-left();
-    }
-    .pull-#{$breakpoint}-right {
-      @include pull-right();
-    }
-    .pull-#{$breakpoint}-none {
-      float: none !important;
-    }
-  }
-}
diff --git a/scss/utilities/_spacing.scss b/scss/utilities/_spacing.scss
index b7ff044b6..99c98f87b 100644
--- a/scss/utilities/_spacing.scss
+++ b/scss/utilities/_spacing.scss
@@ -1,10 +1,11 @@
-// Width
+// Width and height
 
 .w-100 { width: 100% !important; }
+.h-100 { height: 100% !important; }
 
 // Margin and Padding
 
-.m-x-auto {
+.mx-auto {
   margin-right: auto !important;
   margin-left:  auto !important;
 }
@@ -14,18 +15,18 @@
     $length-x:   map-get($lengths, x);
     $length-y:   map-get($lengths, y);
 
-    .#{$abbrev}-a-#{$size} { #{$prop}:        $length-y $length-x !important; } // a = All sides
-    .#{$abbrev}-t-#{$size} { #{$prop}-top:    $length-y !important; }
-    .#{$abbrev}-r-#{$size} { #{$prop}-right:  $length-x !important; }
-    .#{$abbrev}-b-#{$size} { #{$prop}-bottom: $length-y !important; }
-    .#{$abbrev}-l-#{$size} { #{$prop}-left:   $length-x !important; }
+    .#{$abbrev}-#{$size} { #{$prop}:        $length-y $length-x !important; } // a = All sides
+    .#{$abbrev}t-#{$size} { #{$prop}-top:    $length-y !important; }
+    .#{$abbrev}r-#{$size} { #{$prop}-right:  $length-x !important; }
+    .#{$abbrev}b-#{$size} { #{$prop}-bottom: $length-y !important; }
+    .#{$abbrev}l-#{$size} { #{$prop}-left:   $length-x !important; }
 
     // Axes
-    .#{$abbrev}-x-#{$size} {
+    .#{$abbrev}x-#{$size} {
       #{$prop}-right:  $length-x !important;
       #{$prop}-left:   $length-x !important;
     }
-    .#{$abbrev}-y-#{$size} {
+    .#{$abbrev}y-#{$size} {
       #{$prop}-top:    $length-y !important;
       #{$prop}-bottom: $length-y !important;
     }
diff --git a/scss/utilities/_text.scss b/scss/utilities/_text.scss
index 901f77144..b4468e98d 100644
--- a/scss/utilities/_text.scss
+++ b/scss/utilities/_text.scss
@@ -32,6 +32,10 @@
 
 // Contextual colors
 
+.text-white {
+  color: #fff !important;
+}
+
 @include text-emphasis-variant('.text-muted', $text-muted);
 
 @include text-emphasis-variant('.text-primary', $brand-primary);
@@ -44,6 +48,10 @@
 
 @include text-emphasis-variant('.text-danger', $brand-danger);
 
+// Font color
+
+@include text-emphasis-variant('.text-gray-dark', $gray-dark);
+
 // Misc
 
 .text-hide {
diff --git a/scss/utilities/_visibility.scss b/scss/utilities/_visibility.scss
index 63891677e..cdb142869 100644
--- a/scss/utilities/_visibility.scss
+++ b/scss/utilities/_visibility.scss
@@ -1,5 +1,3 @@
-// scss-lint:disable ImportantRule
-
 //
 // Visibility utilities
 //