Replace inline HTML with markdown, rewording, and cleanup.
Closes #18984

Refs #18984
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These are available in _type.scss but not documented.  These classes are useful for styling without adding semantics.

See https://connect.microsoft.com/IE/feedback/details/2263132/edge-ie11-subpixel-problem-with-transparent-borders
Refs #18228
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See http://openradar.appspot.com/24235301
Refs #17497
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https://bugs.webkit.org/show_bug.cgi?id=153224
Closes #17497

https://connect.microsoft.com/IE/feedback/details/2256049/edge-ie11-viewport-rule-has-side-effect-of-making-scrollbars-auto-hide
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the active class should be moved from the list item to the link item

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See https://bugs.webkit.org/show_bug.cgi?id=153056
Refs #18859
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Refs #18782
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see https://github.com/twbs/bootstrap/pull/18782

https://crbug.com/262171 was closed as a duplicate of https://crbug.com/454172

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Fixes #17986
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Currently, there is no actual accessibility page, so this avoids a 404
in the docs. More fundamentally, though, it makes more sense for
accessibility advice to be directly integrated in the docs (which we're
doing, for the most part - though we should ensure that we do
cover/mention everything where appropriate), so there's no need for a
separate accessibility page.

Fixed by way of https://webkit.org/b/152803
Thanks Google Translate (I guess) and WebKit Team!
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Refs #16814
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Refs #18458
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Put all the textual input types right next to each other for easy visual comparison.
Refs #17308, #18763

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- More consistent states and labels in nav examples
- Add stacked pills example for with <nav> element

Refs #18708
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