Remove color from outline reset for improved outlines on focus.

https://github.com/twbs/bootstrap/pull/11551
Remove unnecessary prefixes for gradient mixins given our stated browser support.

https://github.com/twbs/bootstrap/pull/11694
Add line-height to progress bar for proper text alignment within

https://github.com/twbs/bootstrap/issues/10407
Fix indents in modules
Use @screen-xxx variables instead of hardcoded pixel value in dropdown.less
Enable use of form validation class on .radio, .checkbox, .radio-inline, and .checkbox-inline

https://github.com/twbs/bootstrap/issues/10522
Pointer cursor for slider
Add <kbd> element styles to indicate user input via keyboard.

https://github.com/twbs/bootstrap/issues/11193
Add .animation() mixins.

https://github.com/twbs/bootstrap/pull/11244
Add contextual backgrounds

https://github.com/twbs/bootstrap/issues/11572
Prevent the double borders between multiple buttons in an input group.

https://github.com/twbs/bootstrap/pull/11435
Updated .scale() mixin

so that it accepts optional vertical scale as second parameter.
https://github.com/twbs/bootstrap/pull/11748
Add .make-xs-column mixins

https://github.com/twbs/bootstrap/issues/11829
Add and use .text-emphasis-variant() mixin for emphasis classes.

https://github.com/twbs/bootstrap/pull/12247
Reset height on select[multiple] in .input-size() mixin.

https://github.com/twbs/bootstrap/issues/12422