It does work, but I think the problem you might be having is in the way you are setting up your HTML.
Ensure that the <li> elements you want to not be selectable have the unselectable class, as per this example:
<li class="unselectable">unselectable</li>
Fiddle:
http://jsfiddle.net/TtLQa/1/
Also, please refer to this link for browser support information with regards to user-select:none.
Edit:
I just saw your comment, you want to do this via javascript.
Using jQuery, you can easily add or remove a class as you wish:
$(element).addClass("unselectable"); $(element).removeClass("unselectable"); //removes it if it is there, or adds it if it is not $(element).toggleClass("unselectable");
Fiddle:
http://jsfiddle.net/TtLQa/4/
https://stackoverflow.com/questions/826782/how-to-disable-text-selection-highlighting?rq=1