body {
    background-color: gainsboro; 
    color: #444;
    font-family: Arial, sans-serif;
}

#contenedor {
    width: auto;
    border: 10px solid #00f934;
    display: flex;
    flex-direction: row;
    flex-wrap: wrap;
    justify-content: space-between;
}

div.elemento{
    width: 400px;
    border : 10px solid #f90000;
}