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。

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 screen
    and (min-width : 1224px) {
    /* Styles */
    }
    /* Large screens ———– */
    @media only screen
    and (min-width : 1824px) {
    /* Styles */
    }
    /* iPhone 4 ———– */
    @media
    only screen and (-webkit-min-device-pixel-ratio : 1.5),
    only screen and (min-device-pixel-ratio : 1.5) {
    /* Styles */
    }