Programming languages and environments, such as Smalltalk or Java, are sometimes written using themselves. This is often seen as a hallmark of completeness. As you can imagine, those of us building the HTML rendering system for Internet Explorer feel the same way about the browser itself. So in that respect, we can build new HTML elements out of HTML itself.
Prowler is a prototype.
Previously, issues arose about security, and how to implement features that needed to store information, or peek at download information or status on Web pages. The product team was always telling someone how something could be done, but that it really shouldn't be done. Developers needed these features to build more classical applications. In many cases, developers were hosting the WebBrowser OC, and building their UI with HTML, and shipping a stub executable. It was becoming evident that we needed a way to differentiate an application from a Web page.