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.
- Added support for flexbox (set `$enable-flex: true` and recompile) in the grid mixins and predefined classes.
- As part of flexbox, included support for vertical and horizontal alignment classes.
- Overhauled grid mixins to merge `make-col-span` into `make-col` for a singular mixin.
- Added a new `sm` grid tier below `768px` for more granular control. We now have `xs`, `sm`, `md`, `lg`, and `xl`. This also means every tier has been bumped up one level (so `.col-md-6` in v3 is now `.col-lg-6` in v4).
- Changed grid system media query breakpoints and container widths to account for new grid tier and ensure columns are evenly divisible by `12` at their max width.
- Grid breakpoints and container widths are now handled via Sass maps (`$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>`.
- 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](http://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.
- Note that the other features of the plugin (button checkboxes, button radios, single-toggle buttons) have been retained in v4.
- 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.
- 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](http://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.
- Dropped the `.badge` component as it was nearly identical to labels/tags. Use the `.tag-pill` modifier together with the label component instead for that rounded look.
- Tags are no longer floated automatically in list groups and other components. Utility classes are now required for that.
-`.panel-title` to `.card-header`. 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 }}/components/utilities/#font-weight-and-italics)). Note that `.card-title`, while similarly named, produces a different look than `.panel-title`.
- Added `.pull-{xs,sm,md,lg,xl}-{left,right,none}` classes for responsive floats and removed `.pull-left` and `.pull-right` since they're redundant to `.pull-xs-left` and `.pull-xs-right`.
- Added responsive variations to our text alignment classes `.text-{xs,sm,md,lg,xl}-{left,center,right}` and removed the redundant `.text-{left,center,right}` utilities as they are the same as the `xs` variation.
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, use inline styles like `style="display: none;"` and `style="display: block;"`, or toggle the `.invisible` class.
- The old classes (`.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`) are gone.
- They have been replaced by `.hidden-xs-up``.hidden-xs-down``.hidden-sm-up``.hidden-sm-down``.hidden-md-up``.hidden-md-down``.hidden-lg-up``.hidden-lg-down`.
- The `.hidden-*-up` classes hide the element when the viewport is at the given breakpoint or larger (e.g. `.hidden-md-up` hides an element on medium, large, and extra-large devices).
- The `.hidden-*-down` classes hide the element when the viewport is at the given breakpoint or smaller (e.g. `.hidden-md-down` hides an element on extra-small, small, and medium devices).
Rather than using explicit `.visible-*` classes, you make an element visible by simply not hiding it at that screen size. You can combine one `.hidden-*-up` class with one `.hidden-*-down` class to show an element only on a given interval of screen sizes (e.g. `.hidden-sm-down.hidden-xl-up` 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 (e.g. `.hidden-md` is more similar to `.hidden-lg-down` than to `.hidden-md-down`). 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).
