Visual/manual test for checkbox/radio button kbd behavior

Fix typos in documentation - in plural tag names in reboot.md
[skip sauce]

Focus (visually hidden) input radio/checkbox

This allows for keyboard navigation (e.g. arrow keys left/right to
trigger different radio buttons) following an initial mouse click
interaction

js/tests/index.html: Replace jQuery event alias methods with functions that throw errors

js/tests/index.html: Replace jQuery event alias methods with functions that throw explanatory errors, instead of with undefined

Refs https://github.com/twbs/bootstrap/pull/19219#issuecomment-184653699

[skip validator]

CONTRIBUTING: Document restriction regarding jQuery event alias methods

Refs #19192
[ci skip]

Extract $dropdown-min-width variable

[skip sauce]
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Extract $label-pill-padding-x variable

[skip sauce]
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Extract $label-padding-{x,y} variables

[skip sauce]
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Extract $modal-content-border-width variable

[skip sauce]
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Extract $modal-{header,footer}-border-width variables

[skip sauce]
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Extract $modal-dialog-sm-up-margin-y variable

[skip sauce]
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Extract new $modal-dialog-margin variable

[skip sauce]
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Fix docs for new naming of .btn-outline
[skip sauce]

This one was overlooked at edit

See #17719 and #19199

Add extra styles for focused/hovered active outline buttons

v4: Validation states on custom forms

v4: Cleanup after drop csscomb

Remove http://bugzil.la/1060131 from Wall of Browser Bugs

http://bugzil.la/1060131 has been fixed by http://bugzil.la/823483 which should land in Firefox 46.
Refs #14320
[ci skip]

Remove http://bugzil.la/975632 from Wall of Browser Bugs

http://bugzil.la/975632 has been fixed by http://bugzil.la/823483 which should land in Firefox 46.
Refs #10690
[ci skip]

Make translation of custom file input text easier