    FLEX CONTENT
    flex-direction: row; example
    flex-direction: row-reverse; example
    flex-direction: column; example
    flex-direction: column-reverse; example

    flex-wrap: wrap; example
    flex-wrap: nowrap; example
    flex-wrap: wrap-reverse; example

    flex-flow: row wrap; example

    justify-content: center; example
    justify-content: flex-start; example
    justify-content: flex-end; example
    justify-content: space-around; example
    justify-content: space-between; example

    align-items: center; example
    align-items: flex-start;  example
    align-items: flex-end;  example
    align-items: stretch;  example
    align-items: baseline;  example

    align-content: space-between;  example
    align-content: space-around;  example
    align-content: stretch;  example
    align-content: center;  example

    flex-wrap: wrap;  example
    align-content: flex-start;  example

    Total Center  display: flex;
    justify-content: center;
    align-items: center;  example