Refs https://bugzilla.mozilla.org/show_bug.cgi?id=1315994
Refs #21092
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Refs https://bugs.webkit.org/show_bug.cgi?id=162362
Closes #20759
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* #20936 make inline checkbox/radio markup same as stacked

* tweak language and remove extra blank line in snippet

Remove the border-bottom from abbr elements since that's covered with an underline in Normalize.css. Updates the docs to match and tweaks some code comments, too.

fixes #21072: clarify location of variables file (and avoid linking to a file that will be broken depending on the branch name)

* Remove comment that duplicated some code
* Use transition mixin whenever possible
* Create a new function to reduce duplication
* Use the new `breakpoint-infix` method

* Add a color to .card-inverse to account for non .card- prefixed classes

* reword the docs to better articulate what the class does

* Fixed incorrect instruction: "Press ⌘ to copy"

It needed to be "Press ⌘C to copy"

* Updated to concatenate copy string

Updated copy command to concatenated string to prevent potential issues

* Updated the way the copy string concatenates

As per excellent suggestion by @tomlutzenberger

* Removed semicolon line endings

;P

* var not const or let

Add padding to dismiss button for alerts and use position to place it. Removes extra padding on .alert-dismissible's right side, too. We could probably further simplify these things in the future as well.

Solves issue #18946

* Use `breakpoint-min` instead of a counter

* Remove 'xs' from flexbox grid

* Clean up some utilities

- Align CSS properties
- In `_spacing.scss`, we had a comment saying what 'a' was for, but we removed that so this comment no longer applies

* Remove '-xs' from `.navbar-toggleable-xs` and remove duplication

* Fix outdated classes to use newer ones

* Update clearfix to use block instead of table display (also reorder properties for linting)
* update docs snippet for clearfix mixin—was apparently still using Less syntax and had old clearfix hack (even before the block change in this PR)

When the rubberband effect causes Safari to scroll past the top of the
page, the value of scrollTop becomes negative. If the offset of the first
ScrollSpy target is 0 - essentially if the target is at the top of the
page - then ScrollSpy should not clear the active item. Conceptually, the
first item should remain active when rubberbanding past the top of the
page.

This commit fixes issue #21055 by verifying the first scrollspy target is
not at the top of the page before clearing the active nav-item.

* Clean up _flex.scss a little

This commit just cleans up the formating of _flex.scss by changing
it so that it uses a single `@include media-breakpoint-up` instead
of multiple. It also aligns all of the CSS properties so it looks
a bit nicer.

* Remove `-xs` from flex classes

Remove most border-radius properties and instead use overflow: hidden; to properly round corners of a full progress bar. (#21216)

Fixes #19065.