Closes #19906

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Previously, when running the docs locally, the site, rooted at:
    http://localhost:9001/
would reference docs assets using relative URLs such as:
    /../assets/js/vendor/anchor.min.js
which is equivalent to:
    http://localhost:9001/../assets/js/vendor/anchor.min.js
which is nonsense, since the root directory has no parent directory.
Apparently browsers silently ignore this extra '..', hence why this wasn't noticed until now.
But if you adjust Jekyll's `baseurl` setting, this mistake causes incorrect URLs to get generated.

This commit corrects the problem by removing the extra '../' from the paths.

These paths are also referenced in the Gruntfile, where the fix actually allows us to simplify the code.
Previously, in the Gruntfile, we were doing, e.g.:
    path.join('./docs/assets', '../assets/js/vendor/anchor.min.js')
which calculates to:
    ./docs/assets/../assets/js/vendor/anchor.min.js
which can be simplified to:
    ./docs/assets/js/vendor/anchor.min.js
So we can remove the '/assets' suffix from the left argument
and the '../' prefix from the right argument
and still obtain the same result.

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Refs https://bugs.chromium.org/p/chromium/issues/detail?id=454172#c22

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Closes #20165

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Refs http://caniuse.com/#feat=css-nth-child-of
Refs https://drafts.csswg.org/selectors-4/#the-nth-child-pseudo
Refs https://crbug.com/304163
Refs https://bugzil.la/854148

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So remove its entry from the Wall.
Refs #20012, #20024
Refs http://trac.webkit.org/changeset/202324

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Refs #19990
Continues the degruntification process.

Also removes mq4-hover-shim for now,
since it doesn't yet implement the standard PostCSS plugin interface.

Refs https://bugs.chromium.org/p/chromium/issues/detail?id=273306#c34

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Refs #20005
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Refs #17652

Add .flex-*-unordered classes to reset Flexbox `order` CSS property

Fixes #17193: Remove mention and link to non-existant transitions plugin; that intro paragraph was two dupe sentences

Add Wall of Browser Bugs entries for #12832

So remove its entry from the Wall of Browser Bugs
Refs #15832

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Fixes #20002

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Refs #19990

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Fixes #19908

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Refs #19908

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Refs #19908

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Refs #19908

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Refs #19908

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Refs #19908.

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Refs #19908.

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Refs #19908

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Integrate postcss-flexbugs-fixes into build
Refs #18569.