Just to ensure that we are speaking about same things.
Illustrations of vertical/horizontal-align in different flows:
Alignments in flow:vertical a.k.a. <vbox> (attachment "alignment.htm"):
http://www.terrainformatica.com/w3/alignments/flow-v-alignment.png
same document but when div overflow:
http://www.terrainformatica.com/w3/alignments/flow-v-alignment-overflow.png
Alignments in flow:horizontal a.k.a. <hbox> (attachment "h-alignment.htm"):
http://www.terrainformatica.com/w3/alignments/flow-h-alignment.png
http://www.terrainformatica.com/w3/alignments/flow-h-alignment-overflow.png
Note: children of divs are not using any flex dimensions.
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Andrew Fedoniouk
http://terrainformatica.com