More docs accessibility tweaks

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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

Re-add missing "to" in docs

Docs accessibility tweaks: use ``aria-label`` for close ``<button>``, expand advice for ``<abbr>``, form control labels, make sidebar navigation a ``<nav>``

...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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Catch and properly report compile errors that Less.js's toCSS() can throw

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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Update source maps info in README.md.

Remove execute permission on fonts.

Give all embedded live docs examples data-example-id attrs
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This reverts commit 2c1c885e.

Whitelist <li role="separator">...</li>
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Refs https://www.w3.org/Bugs/Public/show_bug.cgi?id=27120
Refs #15263

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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.

Center .icon-prev and .icon-next in carousel

Add .media-object in docs

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