Using Flex/CSS with wkhtmltopdf

This worked for me :

.row {
    display: -webkit-box; /* wkhtmltopdf uses this one */
    display: flex;
    -webkit-box-pack: center; /* wkhtmltopdf uses this one */
    justify-content: center;
}

.row > div {
    -webkit-box-flex: 1;
    -webkit-flex: 1;
    flex: 1;
}

.row > div:last-child {
    margin-right: 0;
}

See https://github.com/wkhtmltopdf/wkhtmltopdf/issues/1522 for more informations.