This page is a malicious page in which user is somehow persuaded to copy stuff to clipboard. The user can use either CTRL/CMD+C or click the "Copy" button below. Then, the code can be pasted in rich editors in other web applications, causing XSS. The XSS to execute is alert(document.domain) to prove the domain it executes in.

Note: for the sake of the demo, CSP disabled is run to show that XSS executes on GMail.