conditional-function-definition.js

var
style = document.createElement('style'),
addRules = function( el, styles )
{ 
  el.appendChild( document.createTextNode( styles ) ); 
};
if ( defined( style.styleSheet ) )
{ 
  addRules = function( el, styles )
  {
    el.styleSheet.cssText += styles;
  };
}

forking-function.js

var
style = document.createElement('style'),
addRules = function( el, styles )
{ 
  if ( defined( style.styleSheet ) )
  { 
    el.styleSheet.cssText += styles;
  }
  else
  {
    el.appendChild( document.createTextNode( styles ) ); 
  }
};