Here’s a look at a handful of the changes since our last alpha:

• 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 non-responsive classes to avoid duplication.
• Added support for jQuery 2.
• And hundreds more Sass improvements, bug fixes, documentation updates, and more.