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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.