Your syntax looks correct to me. How are you testing this? Could it be that you are using IE 10 or above? If so, note that IE 10 and above do not support conditional comments at all, meaning this stylesheet will never be used.
You can always resort to JQuery or Modernizr as an alternative for IE10. For example, you can do
var doc = document.documentElement;
doc.setAttribute('data-useragent', navigator.userAgent);
and in your regular spreadsheet, you can add
html[data-useragent*='MSIE 10.0'] h1 {
color: blue;
}
which will only activate for IE 10. More information on that here.