See issue #25711

As discussed in issue's comment:
https://git.io/vAuPW

Thanks!

This adds `form-group` to add the same vertical space to the `form-check` as in other grouped controls
used in form sample markup.

Thanks!

* Tell where to find the variables to override

* Be consistent with path to _variables.scss

* Get rid of some 'our' in the doc

Replaced by 'Boostrap's' when relevant

* added the styling

* added the documentation

* update for one rule per line

* fix hound error: trailing whitespace

* trimmed off vendor prefixes

* Add note about track and thumb

* Psuedo-elements must be split across multiple rulesets to have an affect

* Fix firefox inner focus

* Seems that FF is the only one affected by this

* Add support for gradients

* Add labels, clarify min/max changes

* add step example

* add custom range vars

For consistency. Closes https://github.com/twbs/bootstrap/issues/25642

[Docs] Clarify suggested use of custom validation styles, add note about resetting form appearance after validation (#25628)

* Expand/clarify recommendation for using custom validation styles

Seems support for correctly announcing browser default navigation bubbles/notifications has improved recently. Now it seems only Chrome (at least on Win and Android, not tested on macOS) is the odd one out.

However, we should really make sure the custom validation messages are also tied correctly to their relevant form controls - need to dig out the PR that will ensure that.

* Add note about resetting form appearance after submission

xref https://github.com/twbs/bootstrap/issues/25626

Closes https://github.com/twbs/bootstrap/issues/25611

2 small typos

* Dropdown - Allow to disable Popper.js style

* Update dropdown.js

* Update dropdown.html

* copy changes

* Remove clip-path from .sr-only utility as it causes perf regressions in Chrome

* change snippet to example

* Add note about insert position of "map-remove"

* Use optional keys instead of required keys

Clarify migration docs to remove mention of .table-responsive being for the parent element vs the table element itself (#25539)

* Prevent focus into closed offcanvas

- Fixes typo with "with at" in "horizontal with at the small breakpoint"

* Move helpful code comments

* Add .carousel-fade option to Sass

* Document example of the fade carousel

* more logical warning of the .active class on carousel items

* Add .dropdown-item-text class

* Add example of plain text within a .dropdown-menu, too

* reorganize, simple/clearer headings

Currently bootstrap only provides 4 variations of `width` and `height` css utilities, which are:

1. 25%
2. 50%
3. 75%
4. 100%.

Here, I'm adding `auto` to the list. This is useful when we need to reset the width/height value
back to the browser default value at some point.

Opera 19 was released in January 2014 and auto-updates so I suggest no need to mention about browser support for this old version.