var winW = 0;

winW = screen.width;
if (winW <= 1600)  {
  document.write('<link rel="stylesheet" type="text/css" href="small.css">');
}
if (winW <= 1280)  {
  document.write('<link rel="stylesheet" type="text/css" href="small.css">');
}
if (winW <= 1024) {
  document.write('<link rel="stylesheet" type="text/css" href="normal.css">');
}
if (winW <= 800) {
  document.write('<link rel="stylesheet" type="text/css" href="small.css">');
}
if (winW <= 640) {
  document.write('<link rel="stylesheet" type="text/css" href="small.css">');
}