Conditional comments are gone in IE 10. That's good. IE 10 is a very good browser. Feature detection is a better way to go in nearly all cases. But what if you find some styling situation where you absolutely need to target IE 10? I guess you'll have to do this.
var doc = document.documentElement;
IE 10's User Agent string is:
Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; Trident/6.0)
Which will result in:
<html data-useragent="Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; Trident/6.0)">