Bootstrap 4.0.0-alpha.2 发布

Bootstrap 4.0.0-alpha.2 发布，更新如下：

Overhauled spacing utilities to use a numerical tiering (to avoid confusion with grid tiers).
Continued refactoring efforts to replace markup-specific selectors with classes across several components (including pagination, lists, and more). Still more to do here with additional components.
Reverted media queries and grid containers from rems to pixels as viewports are not affected by font-size. See #17403 for details. We’ve got a ton of grid work left, too. Feel free to follow along with #18471.
Reverted .0625rem width borders to 1px for more consistent component borders that avoid zoom and font-size bugs across browsers.
Renamed .img-responsive to .img-fluid to avoid future confusion on the various responsive image solutions out there.
Replaced ZeroClipboard with clipboard.js for Flash-independent copy buttons.
Inputs and buttons now share the same border variable to ensure components are always sized similarly.
Updated all pseudo-element selectors to use the spec’s preferred double colon (e.g.,::before as opposed to :before).
Cards now have outline variants and mixins to support extending base classes further.
Utility classes for floats and text alignment now have responsive ranges. This means we’ve dropped the none-responsive classes to avoid duplication.
Added support for jQuery 2.
And hundreds more Sass improvements, bug fixes, documentation updates, and more.