Bootstrap v4.0.0-alpha.6 发布，最后一个 alpha

Bootstrap v4.0.0-alpha.6 发布了，Bootstrap是快速开发Web应用程序的前端工具包。它是一个CSS和HTML的集合，它使用了最新的浏览器技术，给你的Web开发提供了时尚的版式，表单，buttons，表格，网格系统等等。

值得关注的更新：

Flexbox is now on by default, with no fallback! This means we’ve dropped IE9 support, but with significant savings to our code base, simpler components, and improved customization thanks to the power of flexbox.
Rebuilt the navbar with flexbox in mind, leading to some structural changes, but including more flexibility and customization. Includes updated docs, code snippets, more responsive options. All navbars in our examples are also using the latest markup and styles now. Also solves around 10 bugs in the process!
Overhauled flexbox utilities and documented them. Replaced all the previous flexbox utilities with new ones that are named after property: value pairs (instead of creating new names for things). For example, .flex-items-between is now .justify-content-between to apply justify-content: between;. This also adds tons of new flexbox utilities for display, order, wrap, and more.
Revamped navs Sass with flexbox and updated docs. All the floats and clearfixs have been removed, .nav-inline removed (on account of display: flex inlining for us on .nav), added fill and justify options, and more.
Building on the grid improvements from Alpha 5, you can now customize container padding across grid breakpoints and optionally remove grid gutters with .no-gutters.
Similarly, our flexbox grid system has new classes for automatically sizing columns to the size of their content across viewports (e.g., you can now apply col-xs-6, col-md-auto, and col-xl-4 to a single element).
Grid, display, float, margin, padding, and text utility classes have been significantly overhauled. In addition to all classes now being responsive, the xs tier no longer requires a breakpoint abbreviation, making for shorter and more descriptive classes and an easier migration from v3. For example, instead of .d-xs-none, it’s back to .d-none while it remains .d-{sm,md,lg,xl}-none for those breakpoints.
Renamed some .active, all .in, and all .open classes for our JavaScript powered components for more consistent naming. For example, instead it’s now .collapse.show instead of .collapse.in and.show > .dropdown-menu instead of .open > .dropdown-menu to better communicate the visibility of elements.
Refactored carousel with new, simpler styles, better documentation, and less specific selectors.
Rewrote progress component without the <progress> element due to lack of animations, no support for multiple bars, and no support for labels on the progress bars.
Fixed some unescaped #s in our SVG data URLs that are used to add vector background-images.
Renamed .tag back to .badge, like v3, to avoid conflicts with common WordPress styles.
A number of custom form enhancements and bugfixes were made. Label-less custom radios and cehckboxes, fixed SVG URLs in background-images, and more.
Updated Normalize.css to v5.0.0.
.form-check and .form-check-inline now share the same markup. Switch between the two by adding or removing .form-check-inline as a modifier.
Added support for justified navigation when in flexbox mode.
Renamed _animation.scss to _transitions.scss for clarity.
Added new color variables (e.g., $blue) for additional customization.