As per discussion on https://github.com/w3c/wai-wcag-quickref/issues/2
and #13554

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Also, use id rather class name for the navigation, as it's unique and
lets us use correct aria-controls (and without the use of id,
aria-expanded does not get updated correctly by the current dropdown
toggle script)

Before, most callouts hierarchically came under Examples > Four
directions, which is not appropriate. Now they're under Markup

Include callout explicitly mentioning tooltips should only be added to
focusable elements. Closes #16134

Closes #16113
Closes #16114

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Refs #15947

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Closes #15948 by merging it.

Closes #15946 by merging it.

Closes #15915 by merging it.

Fix URL to match the keyword

X-Ref #15742

cc: @mdo

[Fixes #15928] Encourage use of icons with form validation states for colorblind users

Update text per @patrickhlauke's feedback

Update text per @patrickhlauke's feedback.

Fix stupid grammar mistake

Fixes #15106.
Closes #15910.

Added attributes rel and target.