- fix naming of left/right controls
- drop the absolute positioning of things and rely on only 3d transforms
- remove img styles and require classes to avoid inline-block line-height stuff

https://webkit.org/b/150079 has been fixed!
Refs #14975

[ci skip]

So update the Wall of Browser Bugs entry accordingly.

[ci skip]

It's shipping in Chrome 56:
https://bugs.chromium.org/p/chromium/issues/detail?id=231752

[ci skip]

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
[skip sauce]

* 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/
[ci skip]

Refs https://bugs.webkit.org/show_bug.cgi?id=139848
[ci skip]

It's fixed in Firefox 53! Yay!
Refs #12359
[ci skip]

Remove unnecessary .btn-outline and .btn-outline-inverse on docs

Refs https://bugzilla.mozilla.org/show_bug.cgi?id=1315994
Refs #21092
[ci skip]

[ci skip]

Refs https://bugs.webkit.org/show_bug.cgi?id=162362
Closes #20759
[ci skip]

* #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)