hi shashank,
can you explain your problem actually all mobile device change orientation by it self only and browser also do the same the only thing web developer should take care : they should use fluid lay out for robust UI.
and if you wont to load css according to current device orientation then you can use CSS3 media query
http://blog.cloudfour.com/ipad-orientation-css/
also check
http://css-tricks.com/the-perfect-fluid-width-layout/[
^]
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