While working on Mobile Web Application, We can identify orientation or apply device specific css by CSS media queries , but if you need to detect media queries from JavaScript for orientation change, you can observe and notify it with JavaScript to trigger any event or DOM manipulation:
The Key is matchMedia method, it is a part of View Module of CSSOM (CSS Object Model) still in draft mode. http://dev.w3.org/csswg/cssom/

Sources:
http://dev.w3.org/csswg/cssom-view/#the-mediaquerylist-interface
https://developer.mozilla.org/en-US/docs/DOM/window.matchMedia
http://davidwalsh.name/orientation-change

// If there are matches, we're in portrait
if(mql.matches) {  
  // Portrait orientation
} else {  
  // Landscape orientation
}
 
// Add a media query change listener
mql.addListener(function(m) {
  if(m.matches) {
    // Changed to portrait
  }
  else {
    // Changed to landscape
  }
});