The relevant bug has been fixed in Firefox 53:
https://bugzilla.mozilla.org/show_bug.cgi?id=504622

Reboot: Update <fieldset> comment to clarify that the default weirdness is part of the standard. (#21261)

Refs https://html.spec.whatwg.org/multipage/#the-fieldset-and-legend-elements
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* Throw error when a plugin is in transition
* Add unit tests about plugins in transition

Refs https://developer.microsoft.com/en-us/microsoft-edge/platform/issues/7165383/
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Refs https://bugs.webkit.org/show_bug.cgi?id=139848
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It's fixed in Firefox 53! Yay!
Refs #12359
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Remove unnecessary .btn-outline and .btn-outline-inverse on docs

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