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.
- 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.
This list highlights key changes by component between v3.x.x and v4.0.0.
### Reboot
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` change, moving from `rem` to `em` units on many elements, link styles, and many form element resets.
### Typography
- Moved all `.text-` utilities to the `_utilities.scss` file.
- Dropped the `.page-header` class entirely.
-`.dl-horizontal` now requires grid classes, increasing flexbility in column widths.
- Custom `<blockquote>` styling has moved to classes—`.blockquote` and the `.blockquote-reverse` modifier.
- 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 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.
*`@screen-phone`, `@screen-tablet`, `@screen-desktop`, `@screen-lg-desktop`. Use the more abstract `$screen-{xs,sm,md,lg,xl}-*` variables instead.
*`@screen-sm`, `@screen-md`, `@screen-lg`. Use the more clearly named `$screen-{xs,sm,md,lg,xl}-min` variables instead.
*`@screen-xs`, `@screen-xs-min`. The extra small breakpoint has no lower bound, so these variables were logically absurd. Reformulate your expression in terms of `$screen-xs-max` instead.
The responsive utility classes have also been overhauled.
- 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.
- 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).
