DIV响应式

@media only screen and (min-width: 100px) and (max-width: 640px) {
            div {
                width: 100px;
                height: 100px;
            }
        }

@media only screen and (min-width: 641px) and (max-width: 789px) {
            div {
                width: 200px;
                height: 200px;
            }
 }