<link rel="stylesheet" media="all and (orientation:portrait)" href="portrait.css">
<link rel="stylesheet" media="all and (orientation:landscape)" href="landscape.css">

Common CSS Media Queries Break Points
http://stackoverflow.com/questions/16443380/common-css-media-queries-break-points

Media Queries for Standard Devices
http://css-tricks.com/snippets/css/media-queries-for-standard-devices/

Responsive Web Design With Physical Units
http://mobile.smashingmagazine.com/2013/03/21/responsive-web-design-with-physical-units/

Building On The Measure – Logical Breakpoints For Your Responsive Design
http://www.smashingmagazine.com/2013/03/01/logical-breakpoints-responsive-design/

How To Use CSS3 Media Queries To Create a Mobile Version of Your Website
http://mobile.smashingmagazine.com/2010/07/19/how-to-use-css3-media-queries-to-create-a-mobile-version-of-your-website/

Yes, you really can make complex webapps responsive
http://adioso.com/blog/2013/06/responsifying-adioso/

Adapting layout to changing screen orientation
http://stackoverflow.com/questions/13503867/adapting-layout-to-changing-screen-orientation

Responsive background images with fixed or fluid aspect ratios
http://voormedia.com/blog/2012/11/responsive-background-images-with-fixed-or-fluid-aspect-ratios