html-injections

I've got a product that embeds into websites similarly to Paypal (customers add my button to their website, users click on this button and once the service is complete I redirect them back to the original website). I'd like to demo my technology to customers without actually modifying their live website. To that end, is it possible to configure http://stackoverflow.myserver.com/ so it mirrors http://www.stackoverflow.com/ while seamlessly injecting my button? Meaning, I want to demo the experience of using my button on the live website without actually re-hosting the customer's database on my

Assume I have a page with an input box. The user types something into the input box and hits a button. The button triggers a function that picks up the value typed into the text box and outputs it onto the page beneath the text box for whatever reason. Now this has been disturbingly difficult to find a definitive answer on or I wouldn't be asking but how would you go about outputting this string: <script>alert("hello")</script> <h1> Hello World </h1> So that neither the script is executed nor the HTML element is displayed? What I'm really asking here is if there is a standard method of