if (/MSIE (\d+\.\d+);/.test(navigator.userAgent)){ //test for MSIE x.x;
 var ieversion=new Number(RegExp.$1) // capture x.x portion and store as a number
 if (ieversion>=8)
  document.write('<link rel = "stylesheet" type = "text\/css" href = "includes/stylesheet.css" />')
 else if (ieversion>=7)
  document.write('<link rel = "stylesheet" type = "text\/css" href = "includes/stylesheet_ie7.css" />')
 else if (ieversion>=6)
  document.write('<link rel = "stylesheet" type = "text\/css" href = "includes/stylesheet.css" />')
}

else if (/Safari[\/\s](\d+\.\d+)/.test(navigator.userAgent)){ //test for Safari/x.x or Safari x.x (ignoring remaining digits);
 var safversion=new Number(RegExp.$1) // capture x.x portion and store as a number
 if (safversion>=533)
  document.write('<link rel = "stylesheet" type = "text\/css" href = "includes/stylesheet_safari.css" />')
 else if (safversion>=1)
  document.write('<link rel = "stylesheet" type = "text\/css" href = "includes/stylesheet.css" />')
}

else
 document.write('<link rel = "stylesheet" type = "text\/css" href = "includes/stylesheet.css" />')