Bootstrap 4 is a major rewrite of almost the entire project. The most notable changes are summarized immediately below, followed by more specific class and behavioral changes to relevant components.
**Heads up!** This will be in flux as work on the v4 alphas progresses. Until then consider it incomplete, and we'd love pull requests to help keep it up to date.
- Added official support for Android v5.0 Lollipop's Browser and WebView. Earlier versions of the Android Browser and WebView remain only unofficially supported.
- Switched from `px` to `rem` as our primary CSS unit, though pixels are still used for media queries and grid behavior as viewports are not affected by type size.
- As part of flexbox, included support for vertical and horizontal alignment classes.
- Overhauled grid mixins to merge `make-col-span` into `make-col` for a singular mixin.
- Added a new `sm` grid tier below `768px` for more granular control. We now have `xs`, `sm`, `md`, `lg`, and `xl`. This also means every tier has been bumped up one level (so `.col-md-6` in v3 is now `.col-lg-6` in v4).
- Changed grid system media query breakpoints and container widths to account for new grid tier and ensure columns are evenly divisible by `12` at their max width.
- Grid breakpoints and container widths are now handled via Sass maps (`$grid-breakpoints` and `$container-max-widths`) instead of a handful of separate variables. These replace the `@screen-*` variables entirely and allow you to fully customize the grid tiers.
- Media queries have also changed. Instead of repeating our media query declarations with the same value each time, we now have `@include media-breakpoint-up/down/only`. Now, instead of writing `@media (min-width: @screen-sm-min) { ... }`, you can write `@include media-breakpoint-up(sm) { ... }`.
- Dropped the Affix jQuery plugin. We recommend using a `position: sticky` polyfill instead. [See the HTML5 Please entry](http://html5please.com/#sticky) for details and specific polyfill recommendations.
- If you were using Affix to apply additional, non-`position` styles, the polyfills might not support your use case. One option for such uses is the third-party [ScrollPos-Styler](https://github.com/acch/scrollpos-styler) library.
New to Bootstrap 4 is the Reboot, a new stylesheet that builds on Normalize with our own somewhat opinionated reset styles. Selectors appearing in this file only use elements—there are no classes here. This isolates our reset styles from our component styles for a more modular approach. Some of the most important resets this includes are the `box-sizing: border-box` change, moving from `em` to `rem` units on many elements, link styles, and many form element resets.
- Nearly all instances of the `>` selector have been removed, meaning nested tables will now automatically inherit styles from their parents. This greatly simplifies our selectors and potential customizations.
- Responsive tables no longer require a wrapping element. Instead, just put the `.table-responsive` right on the `<table>`.
- Added table header modifiers: `.thead-default` and `.thead-inverse`.
- Renamed contextual classes to have a `.table-`-prefix. Hence `.active`, `.success`, `.warning`, `.danger` and `.table-info` to `.table-active`, `.table-success`, `.table-warning`, `.table-danger` and `.table-info`.
- Dropped `.help-block` and replaced it with `.form-text` for block-level help text. For inline help text and other flexible options, use utility classes like `.text-muted`.
-`.form-group` no longer applies styles from the `.row` via mixin, so `.row` is now required for horizontal grid layouts (e.g., `<div class="form-group row">`).
- The [stateful button](https://getbootstrap.com/javascript/#buttons-methods) feature of the `button.js` jQuery plugin has been dropped. This includes the `$().button(string)` and `$().button('reset')` methods. We advise using a tiny bit of custom JavaScript instead, which will have the benefit of behaving exactly the way you want it to.
- Dropdown toggles no longer require an explicit `<span class="caret"></span>`; this is now provided automatically via CSS's `::after` on `.dropdown-toggle`.
- Renamed the responsive grid modifier classes from `.col-{breakpoint}-{modifier}-{size}` to `.{modifier}-{breakpoint}-{size}` for simpler grid classes. For example, instead of `.col-md-3.col-md-push-9` it's `.col-md-3.push-md-9`.
- Overhauled the grid mixins to merge `make-col` and `make-col-span` into a single `make-col` mixin, thereby ensuring mixins and predefined classes utilize the same float/flex behaviors.
- Added flexbox utility classes for grid system and components.
- Given move to flexbox, alignment of dismiss icons in the header is likely broken as we're no longer using floats. Floated content comes first, but with flexbox that's no longer the case. Update your dismiss icons to come after modal titles to fix.
- The `remote` option (which could be used to automatically load and inject external content into a modal) and the corresponding `loaded.bs.modal` event were removed. We recommend instead using client-side templating or a data binding framework, or calling [jQuery.load](https://api.jquery.com/load/) yourself.
- Dropped nearly all `>` selectors for simpler styling via un-nested classes.
- Instead of HTML-specific selectors like `.nav > li > a`, we use separate classes for `.nav`s, `.nav-item`s, and `.nav-link`s. This makes your HTML more flexible while bringing along increased extensibility.
- Responsive navbar behaviors are now applied to the `.navbar` class via the **required**`.navbar-expand-{breakpoint}` where you choose where to collapse the navbar. Previously this was a Less variable modification and required recompiling.
-`.navbar-default` is now `.navbar-light`, though `.navbar-inverse` remains the same. **One of these is required on each navbar.** However, these classes no longer set `background-color`s; instead they essentiatlly only affect `color`.
- Navbars now require a background declaration of some kind. Choose from our background utilities (`.bg-*`) or set your own with the light/inverse classes above [for mad customization]({{ site.baseurl }}/components/navbar/#color-schemes).
- Given flexbox styles, navbars can now use flexbox utilities for easy alignment options.
-`.navbar-toggle` is now `.navbar-toggler` and has different styles and inner markup (no more three `<span>`s).
- Dropped the `.navbar-form` class entirely. It's no longer necessary; instead, just use `.form-inline` and apply margin utilities as necessary.
- Navbars no longer include `margin-bottom` or `border-radius` by default. Use utilities as necessary.
- All examples featuring navbars have been updated to include new markup.
- Renamed `.label` to `.badge` to disambiguate from the `<label>` element.
- Dropped the `.badge` component as it was nearly identical to labels. Use the `.badge-pill` modifier together with the label component instead for that rounded look.
- Badges are no longer floated automatically in list groups and other components. Utility classes are now required for that.
-`.panel-title` to `.card-title`. Depending on the desired look, you may also want to use [heading elements or classes]({{ site.baseurl }}/content/typography/#headings) (e.g. `<h3>`, `.h3`) or bold elements or classes (e.g. `<strong>`, `<b>`, [`.font-weight-bold`]({{ site.baseurl }}/utilities/typography/#font-weight-and-italics)). Note that `.card-title`, while similarly named, produces a different look than `.panel-title`.
- Overhauled the entire component to simplify design and styling. We have fewer styles for you to override, new indicators, and new icons.
- All CSS has been un-nested and renamed, ensuring each class is prefixed with `.carousel-`.
- For carousel items, `.next`, `.prev`, `.left`, and `.right` are now `.carousel-item-next`, `.carousel-item-prev`, `.carousel-item-left`, and `.carousel-item-right`.
- For prev/next controls, `.carousel-control.right` and `.carousel-control.left` are now `.carousel-control-right` and `.carousel-control-left`, meaning they no longer require a specific base class.
- Made display utilities responsive (e.g., `.d-none` and `d-{sm,md,lg,xl}-none`).
- Added `.float-{sm,md,lg,xl}-{left,right,none}` classes for responsive floats and removed `.pull-left` and `.pull-right` since they're redundant to `.float-left` and `.float-right`.
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.
All `@screen-` variables have been removed in v4.0.0. Use the `media-breakpoint-up()`, `media-breakpoint-down()`, or `media-breakpoint-only()` Sass mixins or the `$grid-breakpoints` Sass map instead.
- The `.hidden` and `.show` classes have been removed because they conflicted with jQuery's `$(...).hide()` and `$(...).show()` methods. Instead, try toggling the `[hidden]` attribute or use inline styles like `style="display: none;"` and `style="display: block;"`.
- All `.hidden-` classes have been removed, save for the print utilities which have been renamed.
- Removed from v3: `.hidden-xs``.hidden-sm``.hidden-md``.hidden-lg``.visible-xs-block``.visible-xs-inline``.visible-xs-inline-block``.visible-sm-block``.visible-sm-inline``.visible-sm-inline-block``.visible-md-block``.visible-md-inline``.visible-md-inline-block``.visible-lg-block``.visible-lg-inline``.visible-lg-inline-block`
- Removed from v4 alphas: `.hidden-xs-up``.hidden-xs-down``.hidden-sm-up``.hidden-sm-down``.hidden-md-up``.hidden-md-down``.hidden-lg-up``.hidden-lg-down`
- Print utilities no longer start with `.hidden-` or `.visible-`, but with `.d-print-`.
- Old names: `.visible-print-block`, `.visible-print-inline`, `.visible-print-inline-block`, `.hidden-print`
- New classes: `.d-print-block`, `.d-print-inline`, `.d-print-inline-block`, `.d-print-none`
Rather than using explicit `.visible-*` classes, you make an element visible by simply not hiding it at that screen size. You can combine one `.d-*-none` class with one `.d-*-block` class to show an element only on a given interval of screen sizes (e.g. `.d-none.d-md-block.d-lg-none` shows the element only on medium and large devices).
Note that the changes to the grid breakpoints in v4 means that you'll need to go one breakpoint larger to achieve the same results. The new responsive utility classes don't attempt to accommodate less common cases where an element's visibility can't be expressed as a single contiguous range of viewport sizes; you will instead need to use custom CSS in such cases.
- Removed the `min--moz-device-pixel-ratio` typo hack for retina media queries
- Change buttons' `[disabled]` to `:disabled` as IE9+ supports `:disabled`. However `fieldset[disabled]` is still necessary because [native disabled fieldsets are still buggy in IE11](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/fieldset#Browser_compatibility).
