[css-flexbox] Expand issue discussion wrt flex-basis: auto

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extra : rebase_source : 5e00f3cb2b82e57fc36734bdb8a8feef14369adb
Fix <hr> sizing

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extra : rebase_source : a9fa61002f3f891c9eeba4a7c05bb63a6457cc92
[css-flexbox] Clean up static-pos section based on converstations with Rossen. (This removes the inconsistency between the two definitions.)

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extra : rebase_source : af4c1e0d9358f165ce2b460ea99f31de231a9637