Why JS Injection isn\'t workin\'?

I\'ve tried a JS sample test in a Website to find a Vuln in a Comment, Like this

But the site shows the script directly as given but not the alert box, why is that so?

Might be down to the browser or a server command/block. I have noticed that firefox doesn\'t support java-script commands in the URL and chrome also is a bit funny about them alot of people are doing them via bookmarklets instead. Also check if you javascript is fully enabled. No script etc.

Well if you use Firefox, there is this browser.urlbar.filter.javascript on about:config that you can set to false, to allow javascript on the url bar.