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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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It has been removed from the HTML specification: https://github.com/whatwg/html/issues/336
Only Presto Opera ever implemented a special UI for it.

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/fyi @mdo

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- add more details on custom checkboxes and radios for how we handle the states and how the css works
- create a disabled custom checkboxes and radios example

Refs #18767
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