media query para responsive

.container {
    width: 800px;
    height: 600px;
}
@media (min-width: 600px) {
    .container {
        width: 600px;
        height: 400px;
    }
}
@media (min-width: 400px) {
    .container {
        width: 400px;
        height: 300px;
    }
}