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 */}