These examples are from the HTMLRockstars post, "Working with the Flexible Box Layout Module (CSS3)"."
This example uses the default box-flex value. While the examples (all of them) use a set width, this is not necessary, it simply makes things easier to illustrate.
This example has all .child boxes set to box-flex: 1; creating equally sized boxes
In this example the first .child is inflexible while .child2 has a box-flex value of 1 while .child3 has a box-flex value of 2 making it more flexible than .child2.
In this example you can see that boxes scale in height to match their siblings. This is the default behavior (box-align: stretch).
In this example you can also see that boxes scale in height to match their siblings, however they start from the bottom of the container using box-align: end;
In this example you can also see that boxes scale in height to match their siblings, however they do not stretch to match the height of their siblings, to do this we set box-align: start;
In this example we've set box-align: center on the parent.
In this example we've set box-align: center and altered the box-flex of .child3 to a value of '2'.
In this example we've set box-align: center and altered the box-flex of .child3 to a value of '2' and .child one to a value of '1' while .child2 remains inflexible.
One of my personal favorites, reversing the display order using box-direction: reverse on the parent container.