You can use the Razor engine to serve a different view based on the caller, but it sounds like your situation can be handled with HTML/CSS.
If IE9 is your only problem, you can use conditional comments and include extra styles/stylesheet.
<!--[if IE 9]>
<style>
#foo { background-color: #fff; }
</style>
<![endif]-->
But for your particular question, you should be able do something even more simple and just modify your stylesheet:
body{
/* all browsers */
background: red;
/* newer browsers overwrite the previous value; old browsers ignore */
background: linear-gradient( to left top, blue, red);
}
IE9 doesn't know what "linear-gradient" means, and so ignores it. This is defined behavior upon which you can rely.
Demo: http://fiddle.jshell.net/K67yd/2/show/