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Closes #15205

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Additional markup for the skip link (to ensure it has the same effective
width as the main content, and to provide an outline around just the
link text) plus some basic styles...make it look a bit more in keeping
with the overall look and feel of the docs.

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- actually add the ``min-width`` extra style (and use ``em`` rather than
``px``) to the 0% and 2% example
- remove the unnecessarily redundant sentence in the "With labels"
section
- remove the unnecessary heading/section specifically for low percentage
case, make it flow naturally as part of the "With labels" section

unless there's a reason why ``tabindex="-1"`` was dropped in there...

Cleaner alternative to using ``<span class="sr-only">``

plus a few suggested approaches (in some cases, elided, as the actual
content of the label - being a generic example - is not important)

to clarify/hint that a ``<label class="sr-only">`` is not the only way
to do it

...and remove the redundant paragraph for basic abbreviation, which
simply repeats what was just said above.

Harmonizing this with same advice given for CSS inline forms

shorter, neater, well supported

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Currently these errors are only reported in the JavaScript console, not in the Customizer UI.
Refs #15296

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Refs #15238
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http://crbug.com/175502 is fixed in Chrome 40 and has been closed.
But Windows is still broken due to http://crbug.com/438064 ,
  which is open and currently targeting Chrome 41.