Most of the designers are always been dependent on the developers for this before

It’s very simple , Use the media attribute to detect the device by its width in pixels and its orientation. if that makes a match it should then be loading the relevant style sheet .

here is how the html should look

<link href="iphone-portrait.css" rel="stylesheet" media="all and (max-device-width: 480px) and (orientation:portrait)" />
<link rel="stylesheet" media="all and (max-device-width: 480px) and (orientation:landscape)" href="iphone-landscape.css" />
<link rel="stylesheet" media="all and (device-width: 768px) and (device-height: 1024px) and (orientation:portrait)" href="ipad-portrait.css" />
<link rel="stylesheet" media="all and (device-width: 768px) and (device-height: 1024px) and (orientation:landscape)" href="ipad-landscape.css" />
<link rel="stylesheet" media="all and (device-width: 800px) and (device-height: 1184px) and (orientation:portrait)" href="htcdesire-portrait.css" />
<link rel="stylesheet" media="all and (device-width: 800px) and (device-height: 390px) and (orientation:landscape)" href="htcdesire-landscape.css" />
<link rel="stylesheet" media="all and (min-device-width: 1025px)" href="desktop.css" />