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.

Update Wall of Browser Bugs accordingly.
Refs #19670

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See https://developer.microsoft.com/en-us/microsoft-edge/platform/issues/7787318/
Refs #19670

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Refs https://developer.mozilla.org/en-US/docs/Web/CSS/:dir
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See https://bugzil.la/1278000
Refs #19670

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See https://bugzilla.mozilla.org/show_bug.cgi?id=1277782
Refs #19839

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