Vertically centering using flex (flexbox)

<style>
.page {
    display: flex;
    border: 5px solid forestgreen;
}
.outer {
    flex: 1;
    display: flex;
    flex-direction: row;
    height: 300px;
    width: 300px;
    border: 3px solid crimson;
}
.inner {
    display: flex;              /* re-starts flex layout */
    flex-direction: column;     /* vertically in a column */
    justify-content: center;    /* center-align children vertically*/
    text-align: center;         /* horizontal center alignment */
    width: 100%;                /* nec for text-alignment */
}
</style>