This page is really dumb and simple: when text is provided as a URL parameter, it will print it. The purpose of this is to be a simple dummy notepad tab that persists by storing information in its URL; this is especially useful in a TreeTabs scenario, where this can create a helper tab to group other tabs under.
It is not meant as an HTML code sandbox. You can use HTML stuff in the content Markdown, but it's sanitized so scripts won't work. (Not that it matters much, since this website doesn't have anything secured for an XSS to target)