Matches the changes from #20874 to the regular select element. This is really only necessary for IE and Edge near as I can tell. Fixes #20810.

Uses a local variable and some calc love to counteract the border-width (times 2) from the height of the select. Fixes #17194 and nullifies #19967.

Avoid the resetting of the already set values in #20397. Fixes #20395.

- Remove margin-bottom override in favor of .mb-0 utility class
- Apply the input line-height styles to match
- Remove the min-height to match the input

Overrides the default width: 100% with an auto. Fixes #20752.

Creating max-width images is not dependent on the display, so setting it is redundant. Cleans up the comments and implementation of the mixin as well.

Fixes #20767

Fixes #18510

Makes it so that the container is no longer wider than the breakpoint used in the media query. This was never really an issue in rendering as the max-width handled it appropriately, but the mismatch was still incorrect.

Fixes #18054

- Fixes issues between float and flex grid systems where container wouldn't fill the available width in Chrome & FF (but would in Safari)
- Fixes #20681
- Fixes #17621 (basically same issue as above issue)

Originally -font-size was added in v3 to set the font-size of the <small> element. Now that we have newer, global type variables, we can use those instead for these kind of things.

Fixes #20859

See https://bugs.webkit.org/show_bug.cgi?id=158342
Fixes #17695

With apologies, copy/paste error following on from trying to fix conflicting Hound/Travis checks for https://github.com/twbs/bootstrap/pull/20821/ slipped through before I noticed them.

* Fixes #20775 without adding extra width and an important flag

* Since we're now getting the extend, we don't need the extra position relative

* rerrange

* getting min-height from the extend already

Only override input border-radius as part of .form-control to avoid bug with iOS Safari (alt fix for #20247) (#20695)

Fixes #20118: Add and use Sass variables to control the color of our embedded SVGs for custom form elements (#20700)

* Fixes #19784: Move from pixels to rems, add a margin-bottom: 0; to keep things aligned

* lintiong

- Fixes #20298 for vertical button group spacing
- Fixes #20784 for horizontal (default) button group spacing
- Alternate fix to #20823 which only applied a fix for vertical button groups

most notable change:
- fixes print/print preview crash in IE11 (caused by `*:first-line`
selector)

* Have same margin for <hN> as a <div> on .card-headers
Related #18726

When you use a `<hN>` as `.card-header` there is `margin-bottom: .5rem` added. The changes set the margin-bottom to 0
* Removed space

* Allow variables input-bg-focus and input-color-focus to be overridden.
* More consistency and control to customize shadows on .form-control:focus