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Closes #15291 by merging it.

Document in the source code that the justified nav is meant for single line list items only. This should help prevent Issues like #15906 in the future.

And add explanatory comments to the Carousel & Dashboard examples regarding holder.js

[skip sauce]

The revert was incomplete.

Fix issue #15601 — Position of X-UA-Compatible, charset and viewport

Add comment to all examples

The non-responsive is correctly saying 2 instead of 3

Fix typo on Utilize

When the screen is shrunk below the md threshold, all the featurette columns stack.
However, since the second featurette is "backwards", when stacked the second featurette
looked awkward as the image came before the heading.

In order to see the changes, you must shrink the browser to below the md threshold.

Use a simple blank.gif and change the background color instead of duplicating stuff.

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plus one example that still used the old <div role="navigation"> and a
fix-up of the callout as per
https://github.com/twbs/bootstrap/issues/15109

Fixed a typo in Sign-in page example
http://getbootstrap.com/examples/signin/

adding explicit aria-expanded="false" initial state (so they're
correctly announced by AT as being collapsed), and - in the case of
old-school use of <a> - role="button"

plus minor tweak to customizer's generated alerts

Closes #14951 by merging it.

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[skip sauce]

Added handling of aria-expanded=true/false to collapse.js, updated
documentation to include advice on making expand/collapse controls
accessible, updated examples and javascript documentation to use
aria-expanded and aria-controls (when targetting single collapsible
element, using ID rather than class selector)

Closes #14147.
Closes #14153.

Fixes #14500