device-pixel-ratio：定义输入设备屏幕的可视宽度与可见高度比率。device-width：输入设备屏幕的可视宽度。
device-width：输入设备屏幕的可视宽度。
orientation ：屏幕横竖屏定向。landscape 是横向，portrait 是纵向【ipad 相反】
/* iPhone 4 ———– */
@media only screen and (-webkit-min-device-pixel-ratio : 1.5),only screen and (min-device-pixel-ratio : 1.5) {/* Styles */}
/* iPads (portrait) ———– */
@media only screen and (min-device-width : 768px) and (max-device-width : 1024px) and (orientation : portrait) {/* Styles */}
/* Smartphones (portrait and landscape) ———– */
@media only screen and (min-device-width : 320px) and (max-device-width : 480px) {
/* Styles*/}
/* Smartphones (landscape) ——— */
@media only screen and (min-width : 321px) {/* Styles */}
/* Smartphones (portrait) ———– */
@media only screen and (max-width : 320px) {/* Styles */}
/* iPads (portrait and landscape) ———– */
@media only screen and (min-device-width : 768px)and (max-device-width : 1024px) {
/* Styles */}
/* iPads (landscape) ———– */
@media only screen and (min-device-width : 768px) and (max-device-width : 1024px)
and (orientation : landscape) {/* Styles */}
/* iPads (portrait) ———– */
@media only screen and (min-device-width : 768px) and (max-device-width : 1024px)
and (orientation : portrait) {/* Styles */}
/* Desktops and laptops ———– */
@media only screenand (min-width : 1224px) {/* Styles */}
/* Large screens ———– */
@media only screen and (min-width : 1824px) {/* Styles */}
